Epsidode 15 – EFI and first start

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.

source: RevZilla

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
  • Throttle position
  • Crankshaft position

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.

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