Quite some time passed after our last epsiode, but we are still here and work on the tractor continues and plans for the upcoming season were made. Basically building the tractor to run the german 3.5T limited modified championship, we were a little sad that the class was only booked for 2 events for the 2020 season. For us this meant, we had to make a choice what to do, if we wanted to run the tractor and collect data and experience. Either we would additionnaly subscribe the tractor for the 3.5T modified or the 2.5T light modified class. Running the 3.5T class would be no problem, but we are lacking lots of power to run that class, so the question was: Can we stay below 2700kg (which we are allowed to run with our engine configuration) to be allowed to run the 2.5T class?
Time to put the tractor and all parts on the scale …
After some math, we think that we can make it. We are actually 30kg below the red line, so that we should be able to make it, without putting everything (and maybe everybody) on a diet, only a few weeks before the start of the season.
The title of this episode suggests, that we will be reporting about the controls on the tractor, so let’s just think what controls will be needed on such a machine. Basically there is no big difference between the basic controls of a car, expect the single rear wheel breaking, as you can find it on stock tractors. But why would you want to break, when you have a 20+ ton sled in your back, who tries to stop you? Well, ass soon as you start to put (enough 😉 )power through the rear wheels into the ground, the front end of the tractor will start to lift and you lose your normal steering. The only way to control the direction in which you are going is by breaking one of your rear wheels, and force the tractor into that direction, similar to driving an excavator or tank. For this purpose, the brakes on the rear axle were changed from drum brakes, which are usually mounted on the planetaries used on our axle, to large diameter disk brakes. We went on and welded flanges to the aluminum planetary housings and mounted laser cut brake disks. The 4 piston hydraulic brake calipers come from a light truck front axle and should provide enough braking force. To control the brakes, we use an individual master clutch cylinder for each side, linked to homemade aluminum pedals.
Of course the tractor is also equipped with a normal steering on the front wheels and a steering wheel. Having build a tractor with a very narrow chassis and a very low center of gravity of the engine, there was no way to pass the engine with some kind of steering column and install a rack and pinion steering. Lots of teams run hydraulic steerings, so that we just wanted to build a similar setup. The basic idea is to just take a hydraulic pump, connect both ends directly to a 2-way hydraulic cylinder and you are good to go. Yeah … that’s just the basic idea and in reality it takes some more tiny bits and tricks to make it work. We ended up with making our own hydraulic cylinder (actually we made two versions, since the first version didn’t provide enough force to turn our wheels), modifying the hydraulic pump (these are basically not made to provide pressure in both directions of rotation), installing one-way valves together with an oil reservoir and having lots of fun to get the air out of the system.
But … we got it to work. It’s always astonishing how such a basically simple problem can take so much time 😉
Now for the must important control of the tractor, throttle control. The throttle has to be controlled by a hand operated, spring loaded lever, which you push into the forward direction. But you are not allowed to just run a wire or rope to your throttle, but the control has to work in both directions. We solved the problem by using a flexible control cable which allows a push/pull operation. Luckily, we were able to scavange this cable from an old fire truck, since these cables are quite expensive. The cable is linked to a homemade adapter, connecting both throttle bodies together.
Actually thinking about the controls, I found, that we need to install some kind of gear shifter …
… so that there is still some work before our first start, which we are planning to do end of April …
Building a vehicle in the size of a tractor, sooner or later you will get confronted with the problem, that you want to be able to drive forward and backward. Knowing that we would face this problem during the planning of this whole endeavour, it was a nice coincidence (or not 😉 ) that Fiat/Iveco relied on Fuller/Eaton gearboxes to cope with the torque of the V8s. Fuller parts and gears are used in a lot of pulling tractors, so having two of these gearboxes should contain enough parts to make our tractor go forward and backward.
Time to have a look into one of these (in)famous Fuller gearboxes.
To much gears, for what we will need, let’s just rip it apart.
First we wanted to use several forward gears, to be able to adjust our wheel speed between runs. But this will just result in extra weight and a further possibility to make a wrong choice for a run. If you don’t have a wrong forward gear, you are not temtded to use it 😉
Not needing that huge housing, we’ve built one out of aluminum plates. Lots of machining to fit the original bearings and tons of drilling and taping.
Since the housing became a lot shorter, the shafts needed shortening and further machining and we needed to find a way to connect the gearbox onto the rear axle.
We were able to reduce the number of gears drastically. Time to explain what will be going on.
The input shaft will drive the top left gear at engine speed, which drives permanently the countershaft and the idler gear.
The output shaft is driven by the sliding sleeve, which allows to change gears (the gearbox is shown in neutral).
The gear on top right turns freely on the output shaft and is permanently driven by the idler gear.
To drive forward, the sliding sleeve is slid to the left and connects the input and output shafts. The output shaft will turn at engine speed.
To reverse, the power will flow through the complete gearbox. The sliding sleev is slid to the right and engages the top right gear onto the output shaft. Through the gears on the countershaft in combination with the idler gear, the output shaft will turn in the opposite direction, compared to the input shaft with a 4:1 ratio.
Moving images are better then a lot of words 😛
Another part of the drivetrain is working, but there are still a lot of parts to be made.
After all the pipes and hoses, one could think that the moment of a first start comes closer. Well, actually the turbo oil circuit didn’t have a pressure regulator yet and the spring in the engine oil pressure regulator was just to weak. Time to machine another pressure regulator into the oil pump housing for the turbos and get a new spring for the pressure regulator in the “see-through” engine oil filter.
We are now able to adjust the oil pressures individually and hope to keep the engine and turbos healthy from a lubrication point of view. Cranking the engine on the starter showed a good build-up of oil pressures, so that this should do the job.
The cardboard model from the previous episode had to be converted into a more fuel resistant version, integrating the fuel tank and the oil tank for the turbos into one piece.
What are we missing for the first start? We gonna need some kind of engine controller for the ignition and fuel delivery to the engine. The devoted followers of this blog will now state that this will all happen with an electronic engine management system or electronic fuel injection (EFI) as it is commonly called. But how does it work, what does it need and could you custom build one of these by yourself?
EFI – How does it work?
Let me start with a small introduction …
The basic principle to get a piston engine to work is pretty simple, should this be your lawn mower or a high displacement alcohol fed pulling engine in our case. Get air and fuel into the combustion chamber, ignite it at the right moment … repeat this process periodically and your engine runs. The trick is, to get the right amount of fuel, matching the amount of air in the combustion chamber, and the timing of the ignition right. For decades all this was done with purely mechanical systems. Carburetors for fuel metering are still around and basically just work on the Venturi effect.
The air aspirated by the engine flows past a nozzle and fuel is pulled with the air into the engine. The size of the nozzle defines the amount of fuel, compared to the amount of air passing your carburetor. The problem with carburetors is, that they become quite complex with large amounts of fuel and air. A solution, which is still quite common in racing, mainly drag racing and tractor-pulling, are mechanical fuel injection systems. The basic idea is, that your engine aspirates a defined amount of air per revolution, defined by the engine displacement. So why can’t we just fit a mechanical fuel pump with a defined displacement per revolution, drive it by the engine and just feed the right amount of fuel per revolution. Sounds simple and works …
… carburetors and mechanical fuel injection systems with their very basic principles have one common enemy … physics, or more precise: fluid mechanics and thermodynamics. Air will just not flow ideally through a pipe with bends, restrictions, sharp corners, … and the amount of fuel your engine needs is not defined trough a volumetric ratio but by a mass ratio. So you need to know the mass of air going into your engine and add the right mass of fuel. As everybody knows from basic physics, the mass to volume ratio of a gas, like air, changes with temperature and pressure … engines tend to get hot, when you compress air with turbos or a supercharger the pressure and temperature rises and air will not freely flow trough a cylinder head with it’s restrictions and past the valves …
… I think you get the point. A good fuel metering system needs to compensate for all these changes in the different operating conditions of an engine. Mechanical systems then tend to become very complex.
*stop it* … this is not a post about carburetors and mechanical fuel systems 😉
What about the ignition timing? That’s an easy one. Basically you just take an electric transformer and connect it with it’s low voltage side to a DC power source. The transformer will store electric energy in a magnetic field. When you open the circuit from the DC power supply, the energy from the magnetic field will discharge over the transformers high voltage side and create a spark at your spark plug to ignite the air/fuel mixture in the combustion chamber. The easiest and a common way since more then a century to switch on and off this transformer (called an ignition coil) is a mechanical switch, operated by a lobe on the engines crankshaft or camshaft and opening the circuit in the desired ignition moment.
If you are running an engine with multiple cylinders, the energy on the high voltage side of the ignition coil is distributed to the individual cylinders by a so called distributor. Handling high voltages in a mechanical distribution system is always fun … *sarcasm off*
So, why should you use an EFI system, when there are well proven and simple solutions? It makes adjusting and tuning your engine just so much easier. A good example are the oil pressure regulators in the beginning of this post. When I want to change any settings or their behavior, it can do this in a small range by turning the adjusting screws, otherwise I have to disassemble it, change the spring, piston diameter or orifice diameters, reassemble it and fine tune everything with the screw. Of course, this is feasible, but time consuming.
Let’s have look how an EFI system does fuel metering and ignition timing. Fuel metering is done through the electric injectors, which are basically just electromagnetic valves. Electric injectors are defined by their flow rate, we are using cm³/minute or thousands of a liter per minute. When you know this flow rate, you can set the exact fuel quantity injected into your engine by opening the injectors for a defined amount of time. On the ignition side, nothing changes, except that the on/off-switching of the ignition coils isn’t done with a mechanical but with electronic switches (MOSFETs or IGBTs), as are the injectors. To avoid a distributor, every cylinder gets his individual ignition coil.
All the magic happens in the engine control unit (ECU). The ECU in it’s basic functions is a simple micro-controller as it can nowadays be found in virtually any device with a battery and more then two push buttons. The micro-controller triggers the injectors and ignition coils based on some basic measurements:
Intake air temperature
Engine temperature (usually coolant temperature)
Manifold air pressure
The ECU knows the engine’s displacement and can calculate the air mass for each engine cycle, since it knows the pressure and temperature of the aspirated air. Additionally you enter the injectors flow rate and your desired air/fuel ratio (14.7:1 for gasoline, 6.9:1 for methanol) and the ECU opens the injector the right amount of time for every engine cycle.
To know the engine speed and position, the ECU relies on a teethed trigger wheel. The wheel misses two teeth, so that the ECU can define the position of the engine and for the rest just counts teeth to the next ignition event.
To get your engine running, that is basically it. Wire everything togther, set your engine and injector parameters up, and you are good to go.
When you have read some or all of our episodes and came to this point in the current episode, you will have asked yourself if we have just bought an EFI system of the shelf or came up with a home brewed solution …
.. you are right, we made everything ourselves. Ignition coils are used ones out of a Mercedes car, injectors come from China and sensors were either parts lying around in the workshop from previous projects or easy available ones from a big online marketplace. The ECU is based on the open-source Speeduino project (www.speeduino.com) and a PCB developped by Krzysztof from Seaside Customs in Poland. Time for some SMD soldering 😉
Tuning of the ECU is done via TunerStudio, usually used in MegaSquirt systems (lots of expensive, freely programmable EFI solutions are based on these).
A problem with automotive sensors is usually that you don’t find their exact specifications and you have to find a way to figure these out. Temperature sensors for example need a 3-point calibration, since they are changing their electrical resistance with temperature in a non-linear way. Let’s assume ambient temperature with 23°C, then we just need to other points. Mother nature came up with two neat temperature points, everybody can recreate. Water changes it’s state from liquid to solid at 0°C and goes from liquid to vapor at ~100°C (have to love our temperature scale 😉 ) Time to boil some water, put your sensors in, and repeat with ice water. Note the individual resistance values and you are done. No need for fancy test equipment.
After everything was together, time to crank it …
We had checked the correct ignition timing before, but the engine just kept doing single pops and then lost the crankshaft signal. After checking all signals and the correct function of the ECU, we found the solution in a faster cranking speed.
We didn’t really adjust the fuel pressure before, so that the fuel pressure went a little high after the mechanical fuel pump started feeding, resulting in a really rich setting. Amazing, how fast you can transform 6-8 liters of methanol into noise an this distinctive sweet smell 😉
As one can see, we were not really prepared for a start, but were able to check all oil pressures and are happy that these came out just as expected … thanks to the homemade pressure regulators 😉
Now that the engine is running, comes the time for all the fine tuning of the ECU, but I think we will cover this in another episode. The drive line and gearbox are in the making, but we decided to take our time and not finish everything in a hurry, just to get to a pull this year. We wouldn’t be pleased with the result of that kind of work and prefer to start well prepared in the next season.
People visiting us regularly in the workshop don’t see big changes on the tractor and come to the point where they ask what we are doing all the time, since the tractor is basically ready …
All the major components are painted and back on the frame, engine, turbochargers, rear axle, … . So, we should be good to start it up an take it to the first pull. But it’s missing a lot of the small details, which are mainly related to connecting everything together. Time to get you a small insight, what has to be interconnected on such a machine:
The engine needs to be connected to the rear wheels through drive shafts. No U-joints allowed, so everything has to be perfectly aligned and will be connected with splined shafts and couplers accordingly. Since you can’t buy these of the shelf, they need to be machined. They have to take all the horsepower and you can’t just do them out of normal steel, so you need to find an appropriate material. The solution comes in the form of tool steel with the desired hardness and flexibility at the same time. The disadvantage of tool steel is, that you will have a hard time to machine it. Lots of researches and tips from other competitors lead us to a type of steel which comes in a normalized form, thus is easy to machine and will get it’s final mechanical characteristics in a specific hardening process.
Time to machine the outside and inside splines …
Each spline has to be cut individually, indexing the splines has a zero-error-tolerance and you often need to take several passes to come to your final dimensions and have a perfect fit … time consuming and you have a lot of time to think about all the other work in front of you to get the tractor done 😛
… all major drive line parts are finished and on their way to a specialized company for hardening. Finding a company which can and will harden the parts to the needed specifications in our area is a whole other story
… still missing the long drive shaft between the clutch and gearbox. It needs some final touches on the gearbox (good topic for another episode 😉 ), to get final dimensions. It will anyways be machined out of another material, since there is no way (at our hands) to harden such a long piece without a tremendous amount of deformation.
In parallel we are making all the fluid and gas connections. Let’s try to list all the needed gas (air, exhaust, breathers, …) connections, starting with the trivial ones:
Intake air to the turbochargers … easy, since there is just an opening to free air … well, NO. We need to run an air restrictor in our class and ETPC rules call for an intake protection (I think, I will cover all the safety stuff in a separate episode).
Exhaust gasses to free air … no problem neither, just a big pipe with an elbow and you are done … well, NO. There a rules for your exhaust pipes and further safety elements behind the turbos.
Compressed air goes from the turbochargers into the engine … sounds easy, some pipe, rubber hose and clamps … well, NO. In our case we didn’t want to go with full custom made intake manifolds from the beginning, so we modified the stock aluminum intake manifolds, welded some pipes and elbows to it and were good to go. Besides this, the connection between the turbos and the engine had to incorporate the following elements: throttle bodies, blow-off valves, holes (actually it was a little more complicated) for the fuel injectors, connections for temperature and pressure sensors, ports for vacuum connections (other elements of the engine management system need these), … I certainly missed some others 😉
Exhaust manifolds from the engine to the turbochargers … sounds easy and needs just hours of work in stainless steel … cutting, welding, grinding an press forming adapters from a specific round to a specific rectangular shape
… at some point you need to integrate connections to be able to disassemble your construction 😛 V-band connections are great for this purpose
The turbo chargers, due to their basic function principle with hydrodynamic bearings (the shaft spins and is centered in a pressurized oil layer in the bearings) need their own oil supply and return, which we realize with a dedicated oil pump, filter and a separate oil tank for the turbos.
All tanks, in fact circuits with liquids, need a ventilation with a receptacle to catch any liquid exiting the ventilation and avoid contamination of the environment. Counting all these gives a nice number as well: rear axle, rear planetaries, gearbox, two braking circuits, hydraulic steering, fuel tank, turbo oil tank, crankcase ventilation, …
Hydraulic steering and brake systems … I’ll cover these and their function in a separate episode with the other controls of the tractor (clutch, throttles, shut-offs, …)
As we switched the engine lubrication system to an external oil pump, oil needs to go from the crankcase to the oil pump and from the oil pump into the engine … easy … well, NO. Connections were welded into the oil pan, after the oil pan was shortened and thus allow the oil feed to the two stage oil pump.
Actually they are not just pipes welded to the oil pan, but inside they are routed to the deepest point of the pan, incorporating filtering elements to avoid pump damage … hard to get a picture of it now
Not using the original oil pump left us with the question how to get the oil into the engine. We ended up feeding the oil directly into the main oil galleries of the engine. Oil supply in an engine is just a work of art, when you consider that all the complete internal oil supplies can’t be cast with the engine block, but are machined and drilled afterwards.
Not using lots of the stock oil supply, we ended up without an oil filter and oil pressure regulator. The solution came in machining an oil filter according to our needs, integrating an adjustable pressure regulator with a return line into the crankcase. We integrated another feature, which is a transparent plate, in the filter, allowing us to inspect our filter element without the need to open the filter. This allows for a quick check on engine health between runs and eventually call it quit before a major engine damage.
Most of the hose connections are done with AN-fittings (Dash-fittings or whatever you are used to call them). This is an amazing system, common in motorsports, which allows you the realization of virtually any connection without the need of press fittings. Well, nearly any connections, besides those which are non-standard or you just forgot to order the correct ones … let’s just machine them
With all the connections, the front of the engine becomes really cramped
Then comes all the fuel stuff … fuel pump, fuel filter, fuel pressure regulator, fuel distribution, and fuel rails. I’ll try to cover this in a dedicated series of episodes on engine management.
What did I miss? … Aaah, all the electrical stuff
I think, one gets a pretty good overview what was and is ongoing in the workshop, without seeing major changes in the state of the tractor.
Talking about liquids, some of them have their containers, trough the way how they are constructed, as the engine oil or the rear axle oil, others need to have their containers build. With not a lot of space left on the front, due to overall vehicle length, time for old-school cardboard models 😉
Building a pulling tractor with a big engine is like getting yourself a large pet … lets say: a bear 🤣
It’s amazing to have one, but at a certain point you need to feed it and fulfill all it’s basic needs 😉
In one of the previous episodes, we showed how we will feed the engine with air … the turbos will keep care of this. Having lots of air, you need lots of fuel and this fuel needs to get pumped somehow. Electrical fuel pumps have two disadvantages in our application: they don’t deliver enough fuel and they consume lots of electrical energy, creating the need to somehow get this electrical energy. The solution lies in a mechanically driven fuel pump, suited for methanol, as they are commonly used in racing.
Nice little pump, has metric bearings in it but someone decided tho have an imperial hex shaft to drive it 🤐
A more basic need is the oil supply of your engine. As explained before, we wanted to use the stock oil pump in the engine, but were made aware (several times) not to do so. Investigating all this showed, that the original one stage, big displacement oil pump spins faster than the crankshaft, pumping to much oil at high engine speeds, combined with the risk of cavitation and the forming of oil foam. The main and connection rod bearings wouldn’t appreciate that foam and their life expectation would decrease drastically. So … how do other people tackle this issue? The solution lies in the use of an external oil pump and commonly people use specific modified, three stage, dry sump oil pumps, as they are common on high performance racing engines. For us the terms “motorsport”, “racing” and “high performance” are always related with high costs for solutions which don’t seem to be that much of rocket science as one would expect for the price. Not willing to spend that much money on “just an oil pump”, it was time to use the might of the internet to find out about the specifications of these expensive pumps. Well, long stories short, we opted for a three stage hydraulic pump, as they would be used on excavators and other industrial machines, with the same flow rates as the dry sump racing pumps, commonly used. We have no weight issue, and we will enlarge the pump ports, so that the pumps can flow without restrictions.
But why three pump stages? As mentioned before, single large displacement pumps appear to be problematic, so that the engine oil supply is split between two smaller pump stages. The third pump stage will be used to feed oil to the turbochargers in a completely separate oil system. This will avoid contamination and allow us to run different oils, best suited for methanol engines and lubricating turbochargers, without making compromises. I think, oil will be worth a complete episode on it’s own 😉
Both pumps, methanol and oil, are mechanically driven … how do we drive them from the engine? The solution comes with a system, based on a timing belt …
Everything will be driven from the front of the crankshaft, no problem. Well … we have to consider transmission ratios an everything has to fit between the chassis rails, due to the very low mounting position of the engine … total width of only 44cm 🤔
For the transmission ratios, this meant the pulley on the crankshaft has to be as small ass possible, compared to a large pulley on the oil pump. The methanol pump should be fine with a 1:1 ratio, since the flow rate fits the target engine speed and maximum capacity of the used injectors quite well.
We absolutely wanted to go with one of these aircraft, direct crank drive starters, so this has to fit somehow with our pulley system. Time to find out, what we can machine, on purely conventional machines out of chrome-molybdenum steel.
Some other, minor stuff 😜, had to go on the front of the crankshaft as well. The EFI system needs a trigger signal at crankshaft speed, which we will get from a toothed wheel. In our application, this trigger wheel needs to clear a pretty large external crankshaft counterweight. The tractor will get some electrical consumers, mainly the EFI system, and we don’t want to solely rely on battery power … a belt pulley has to go on the crankshaft and we will try to find a nice spot (or just space which is not yet overfilled) to mount an alternator.
Timing belts have a lot of advantages, but are limited in the maximal torque which they can transmit per single tooth on a pulley. Therefore you need a minimum number of teeth from a pulley which engage into the belt at any moment to transmit the needed energy from belt to the pulley or vise-versa. Using small pulleys, we were able to overcome this issue by routing the belt in a serpentine pattern in combination with the needed tension pulley.
With the pulley and belt system in place, time to find out if everything still fits in front of the engine.
After all the machining, time to think about connecting everything with hoses, content for another episode 😉
With a project like this, it is amazing to see other peoples interest in your work and progress. Wherever you come, people ask about the current state and when the tractor will run for the first time.
You can prepare the nicest project plans but often it is just not in your hands. We were waiting to get laser cut parts delivered, which took 7 weeks instead of the usual two weeks, so that we were blocked in our progress on some parts. It came to a point were we even started to paint the first components … watching paint dry is definitely not one of our favorites 😉
Be assured, the tractor will not come completely in orange paint. We came up with a particular color scheme for the tractor, where orange is just one part.
Talking about organizing parts, the material value of a tractor is only a minor part, compared to the value of the work going into such a project. Work which is done by the entire team, starting with cleaning parts, assembling/disassembling, measuring and finally designing, welding and machining all the parts. Not having a high-tech tool and machine shop with CNC controlled machines at our hands, every part is done the old-school way on conventional lathes and mills. In a team which started as a bunch of friends, everybody is learning skills from each other to operate the machines and is thus able to realize even more complex parts on his own. The principles in the workshop are clear: Don’t stop were you are, you grow with your challenges and if there is no solution, it is not a problem.
I think this is a good place to thank everybody for their motivation, devotion to the project, the time invested so far an the time you will invest to get the tractor running. Another big thanks has to go to our families, girlfriends and so on, allowing us to invest so much time in our project, without their support it would just be impossible.
I missed another principle … if, after a long day at the job and a whole evening working on the tractor there is not a lot of motivation left, or something just didn’t work as expected, there is always time for a beer or two between friends.
What about the small steps?
Front and rear axles with the steering and brakes are done and just awaiting their final paint job.
The clutch and flywheel are ready and fit perfectly. A big thanks to Freakshow Performance for the amazing work.
We finally found a piece of large diameter steel pipe to get the clutch protection done. Looks like some more machining work 😉
The auxiliary drive for the oil- and fuel pump, incorporating the direct crank starter and trigger wheel for the EFI, comes together nicely.
Spark plugs are in and ignition coils mounted to the cylinder heads.
Fuel injectors fit the intake manifolds and we are currently machining the fuel rails.
Turbos and throttle bodies are on and we are working on the intake and exhaust piping.
The gearbox housing is getting machined and the Fuller parts are getting the needed modifications.
Decisions are made on the material for the drive shafts … some more machining.
Seems like time to get back to the workshop. We are currently to busy to keep up with the technical episodes, but these will follow, with the tractor running. To close this episode, here is a picture what the italian power pack actually looks like 😉
Let’s just start with a statement of Jeremy Clarkson, which shows the basic understanding of most people about turbochargers:
“A turbo: exhaust gasses go into the turbocharger and spin it, witchcraft happens and you go faster.”
So before we show our turbo setup, which is currently still in the making, we thought it would make sense to go into the basics of turbocharging an engine.
As stated in a previous episode, making more power out of your engine is just possible when you are able to burn more fuel. But you can just burn more fuel, when you get more air into your engine. Usually an engine is fed with air by means of the atmospheric pressure, which pushes the air into the combustion chamber(s). The amount of air for combustion is theoretically your engine’s total displacement for every two engine revolutions on a 4-stroke engine. This amount is limited trough airflow losses while the air flows into the combustion chamber(s) and usually further decreases with engine speed, since the air has less time to get into your combustion chamber.
To get more air into your engine, as trough atmospheric pressure, you have to force it in by increasing the pressure, or creating boost as tuners would call it . This is usually done by adding an air pump in form of a compressor. This compressor needs to flow large amounts of air without having to dramatically increase the pressure. Centrifugal compressors are a good choice for this task, working on a basic principle without a huge number of moving parts or needing complicated sealing.
To drive such a compressor, you need mechanical energy. This mechanical energy can come directly from your engine via a special gearbox, stepping up the engine speed, to drive the impeller. Another option is to use the energy out of the exhaust gasses to drive the compressor, as it is done in a turbocharger.
In a turbocharger, the centrifugal compressor is directly coupled (on a common shaft) to a turbine wheel. The turbine wheel is driven trough the energy still available in the exhaust gasses and transferring this energy to the compressor wheel and you can force more air into your engine.
In theory, this is pretty simple, but when it comes to choosing the right turbocharger, it gets a little more complicated, since the offer in turbochargers is huge. Basically you can make the choice for a compressor, based on horsepower numbers you “want” to make. Turbo manufacturers give you the numbers for the maximum amount of air, their compressors will flow. This is nice, but will you be able to get all that air through you engine? This is, where pressure an turbo speed come into the game. Two basic theories:
Your engine can not flow the air, the pressure increases, the compressor wheel slows down, less air is moved, …
Your engine can not flow the air, the pressure increases, the turbine delivers enough energy to keep the compressor spinning, the air heats up, boost pressure rises but you are not making more power, …
The two reflections lead to two extremes:
The compressor wheel slows down to a point where it won’t make any gain in pressure.
The compressor wheel over-spins and will not be able to make any gain in pressure neither.
Therefore, turbo manufacturers provide compressor maps for compressors.
The horizontal axis shows the mass-flow of air, thus somehow proportional to the power you can make.
The vertical axis is the pressure ratio, compared to atmospheric pressure.
The islands in the map show the compressor efficiency.
The dotted lines show compressor speed in rounds per minute.
Everything on the left of the islands is above the surge limit of your compressor wheel. Basically your compressor wheel is to huge and you cannot keep it spinning with the energy you provide trough the exhaust turbine.
On the right of the islands, your compressor spins but you are not able to get the air through your engine and compressor efficiency drops. You are just producing hot air
As an optimum, you want to operate your compressor in the center islands. When you try to follow the basic thoughts, you come to the conclusion, that the compressor operation is in direct relation with the exhaust turbine, providing the energy to the compressor. Therefore, you can combine a given compressor with different exhaust turbines and exhaust turbine housings to make best use of the energy in your exhaust gasses for you application. There is a huge difference between low end torque for street driven vehicles and WOT (wide-open-throttle) as used in tractor-pulling.
Our class is limited with air restrictors, which every competitor has to use. The aim is to limit the maximum power for everyone to about the same amount. Running two turbos, we have to use two 76mm (~3 inch) restrictors. Our first thoughts were to just go with compressors of exact this size. But since we have to use the restrictors anyways, we decided to go with slightly bigger compressor wheels an target for high efficiency. Based on availability and budget, the choice was made to go with S400 clones equipped with 88mm billet compressor wheels.
Comparison with a standard turbo out of a 1.9TDI engine 😉
Based on the thought, that burning methanol in a high displacement engine will generate huge amounts of exhaust gasses, we went for the biggest exhaust wheel and housing combination available for this compressor.
Speaking about exhaust gas volume. What do you do with excess exhaust gasses, or can you even control the amount of exhaust gasses to the turbine of your turbocharger? Of course you can 😉
Controlling the amount of exhaust gasses to your turbo allows you to control the energy you provide to the compressor, and thus you can control the amount of air the compressor flows. Basically you are using a mechanical valve to bypass exhaust gasses before the turbine, which will not drive the turbine. This valve is called the wastegate.
Wastegates are eather integrated in the turbochargers turbine housing, called an internal wastegate or, as shown in the image above, they are positioned on the exhaust manifold before the turbocharger, called an external wastegate.
Basic wastegate control is done trough the wastgate spring, which counteracts against the exhaust gas pressure. The wastegate opens progressively when the exhaust gas pressure rises, reducing the energy delivered to the turbocharger and thus regulating the boost pressure generated by the compressor. This is just a very basic control and most wastegates are build with an additional pneumatic control, having a membrane under the spring, so that you can add force to the spring or counteract against it by applying pressures trough the two additional control ports. These pressures can either be directly your boost pressure or controlled trough your engine management system.
But what about the “choo-chooo-chooo” in this episodes title. This describes the distinctive sound, lots of people directly relate to a turbocharged engine. But it is neither generated by the turbocharger, nor the wastegate. This sound is generated, and often ridiculously amplified, in the so called blow-off valve. A turbocharged system does not necessarily need this valve and one usually does not find it on Diesel powered engines and low boost gasoline applications. But why is it needed and extensively used on tuned, mostly spark ignited engines?
The need comes from the fact, how the engines power delivery is controlled. Generally speaking this is throttle control. On a spark ignited engine, you have a throttle, which controls the airflow to the engine. This throttle is placed between the compressor and the engine’s intake. A Diesel powered engine is controlled through the amount of fuel, the fuel injection pump delivers, thus it does not need a throttle controlling the airflow.
Now the blow-off valve’s main purpose is to prevent pressure peaks and avoid excessive wear of a turbo charger. The pressure peaks are generated, in a spark ignited engine, when you shut your throttle after high boost operation. With the throttle shut, the exhaust gasses are still delivering energy to the turbine and the compressor pumps air. The throttle allows just a marginal amount of air to travel to the engine and the pressure between compressor and throttle body rises, well over the surge pressure of the turbocharger and excess air would push back out of the compressor intake. This results in high thrust bearing wear or failure of your turbocharger.
Basically the function of a blow-off valve is identical to that of the wastegate. The blow-off valve is positioned, as a spring loaded valve, in the intake manifold, between the compressor and throttle body. To help the opening of the valve, the pressure chamber in the valve containing the spring, is connected to the intake manifold, between the throttle body and the engine. Shutting the throttle results in a vacuum between the throttle body and the engine and thus helps in the opening of the blow-off valve. Releasing pressure through the blow-off valve sometimes results in an oscillation of the blow-off valve, resulting in the distinctive “choo-chooo-chooo” sound 😉
A basic turbocharger setup then looks as in the following picture. The picture shows no intercooler. We will do intercooling with additional fuel or water injection (topic for another episode 😉 )
Our system will be setup exactly like this. Well, we have 8 cylinders, 2 turbos, 2 throttles, 2 wastegates, 2 blow-off valves …
The plan is to build a separate turbo setup for each side of the engine. Time to figure out, where we will fit all that stuff …
A mock-up is a good idea to find out if everything works as expected, but after all we still imported everything into CAD to design the needed flanges and supports. Oh, and you need some piping 😉
Time to get all the parts build and we will keep you updated in the following episodes.