Besides being your run of the mill computer geek, I’ve always been a bit of a car geek as well. This often solicits down-the-nose looks from others who associate such people with V8 Supercar lovin’ petrolheads, which has always surprised me little because the most fun parts of working on a car are all just testing physics theories anyway. With that in mind, I’ll do this writeup from the point of view of the reader being a non-car, but scientifically minded person. First a bit of background…
For the last 3 years or so my dad and I have been working a project to fuel inject our race car. The car itself is a 1968 Mk2 Cortina and retains the original 40 year old 1600 OHV engine. This engine is originally carbureted, meaning that it has a device that uses the vacuum created by the engine to mix fuel and air. This mixture is crucial to the running of an engine as the ratio of fuel to air dramatically alters power, response and economy. Carburetors were used for this function for a long time and whilst they achieve the basics very well, they are, at best, a compromise for most engines. To overcome these limitations, car manufacturers started moving to fuel injection in the 80’s, which allowed precise control of the amount of fuel added through the use of electronic signals. Initially these systems were horrible however, being driven by analog or very basic digital computers that did not have the power or inputs needed to accurately perform this function. These evolved to something useful throughout the 90’s and by the 00’s cars were having full sequential system (more on this later) that could deliver both good performance and excellent economy. It was our plan to fit something like the late 90’s type systems (ohh how did this change by the end though) to the Cortina with the aims of improving the power and drivability of the old engine. IN this post I’m going to run through the various components needed from the electrical side to make this all happen, as well as a little background on each. Our starting point was this:
To have a computer control when are how much fuel to inject, it requires a number of inputs:
- A crank sensor. This is the most important thing and tells the computer where in the 4-stroke cycle (HIGHLY recommended reading if you don’t know about the 4 strokes and engine goes through) the engine is and therefore WHEN to inject the fuel. Typically this is some form of toothed wheel that is on the end of the crankshaft with a VR or Hall effect sensor that pulses each time a tooth goes past it. The more teeth the wheel has, the more precisely the computer knows where the engine is (Assuming it can keep up with all the pulses). By itself the toothed wheel is not enough however as the computer needs a reference point to say when the cycle is beginning. This is typically done by either a 2nd sensor that only pulses once every 4 strokes, or by using what’s know as a missing tooth wheel, which is the approach we have taken. This works by having a wheel that would ordinarily have, say, 36 teeth, but has then had one of them removed. This creates an obvious gap in the series of pulses which the computer can use as a reference once it is told where in the cycle the missing tooth appears. The photos below show the standard Cortina crankshaft end and the wheel we made to fit onto the end
To read the teeth, we initially fitted a VR sensor, which sat about 0.75mm from the teeth, however due to issues with that item, we ended up replacing it with a Hall Effect unit.
- Some way of knowing how much air is being pulled into the engine so that it knows HOW MUCH fuel to inject. In earlier fuel injection systems this was done with a Manifold Air Flow (MAF) sensor, a device which heated a wire that was in the path of the incoming air. By measuring the drop in temperature of the wire, the amount of air flowing over it could be determined (Although guessed is probably a better word as most of these systems tended to be fairly inaccurate). More recently systems (From the late 90’s onwards) have used Manifold Absolute Pressure (MAP) sensors to determine the amount of air coming in. Computationally these are more complex as there are a lot more variables that need to be known by the computer, but they tend to be much more accurate for the purposes of fuel injection. Nearly all aftermarket computers now use MAP and given how easy it is the setup (just a single vacuum hose going from the manifold to the ECU) this is the approach we took.
The above are the bare minimum inputs required for a computer to control the injection, however typically more sensors are needed in order to make the system operate smoothly. We used:
- Temperature sensors: As the density of air changes with temperature, the ECU needs to know how hot or cold the incoming air is. It also needs to know the temperature of the water in the coolant system to know whether it is running too hot or cold so it can make adjustments as needed.
- Throttle position sensor: The ratio of fuel to air is primarily controlled by the MAf or MAP sensor described above, however as changes in these sensors are not instantaneous, the ECU needs to know when the accelerator is pressed so it can add more fuel for the car to be able to accelerate quickly. These sensors are typically just a variable resistor fitted to the accelerator.
- Camshaft sensor: I’ll avoid getting too technical here, but injection can essentially work in 2 modes, batched or sequential. In the 4 strokes an engine goes through, the crankshaft will rotate through 720 degrees (ie 2 turns). With just a crank sensor, the ECU can only know where the shaft is in the 0-360 degree range. To overcome this, most fuel injection systems up to about the year 2000 ran in ‘batched’ mode, meaning that the fuel injectors would fire in pairs, twice (or more) per 720 degrees. This is fine and cars can run very smoothly in this mode, however it means that after being injected, some fuel mixture sits in the intake manifold before being sucked into the chamber. During this time, the mixture starts to condense back into a liquid which does not burn as efficiently, leading to higher emissions and fuel consumption. To improve the situation, car manufacturers starting moving to sequential injection, meaning that the fuel is only ever injected at the time it can go straight into the combustion chamber. To do this, the ECU needs to know where in 720 degrees the engine is rather than just in 360 degrees. As the camshaft in a car runs at half the crankshaft speed, all you need to do this is place a similar sensor on this that produces 1 pulse every revolution (The equivalent of 1 pulse every 2 turns of the crank). In our case, we had decided that to remove the distributor (which is driven off the crank) and converted it to provide this pulse. I’ll provide a picture of this shortly, but it uses a single tooth that passes through a ‘vane’ type hall effect sensor, so that the signal goes high when the tooth enters the sensor and low when it leaves.
- Oxygen sensor (O2) – In order to give some feedback to the ECU about how the engine is actually running, most cars these days run a sensor in the exhaust system to determine how much of the fuel going in is actually being burned. Up until very recently, virtually all of these sensors were what is known as narrowband, which in short means that they can determine whether the fuel/air mix is too lean (Not enough fuel) or too rich (Too much fuel), but not actually by how much. The upshot of this is that you can only ever know EXACTLY what the fuel/air mixture is when it switches from one state to the other. To overcome this problem, there is a different version of the sensor, known as wideband, that (within a certain range) can determine exactly how rich or lean the mixture is. If you ever feel like giving yourself a headache, take a read through http://www.megamanual.com/PWC/LSU4.htm which is the theory behind these sensors. They are complicated! Thankfully despite all the complication, they are fairly easy to use and allow much easier and quicker tuning once the ECU is up and running.
So with all of the above, pretty much the complete electronics system is covered. Of course, this doesn’t even start to cover off the wiring, fusing, relaying etc that has to go into making all of it work in the terribly noisy environment of a car, but that’s all the boring stuff
Finally the part tying everything together is the ECU (Engine Control Unit) itself. There are many different types of programmable ECUs available and they vary significantly in both features and price, ranging from about $400 to well over $10,000. Unsurprisingly there’s been a lot of interest in this area from enthusiasts looking to make their own and despite there having been a few of these to actually make it to market, the most successfully has almost certainly been Megasquirt. when we started with this project we had planned on using the 2nd generation Megasquirt which, whilst not having some of the capabilities of the top end systems, provided some great bang for bang. As we went along though, it became apparent that the Megasquirt 3 would be coming out at about the right time for us and so I decided to go with one of them instead. I happened to fluke one of the first 100 units to be produced and so we had it in our hands fairly quickly.
Let me just say that this is an AMAZING little box. From what I can see it has virtually all the capabilities of the (considerably) more expensive commercial units including first class tuning software (Multi platform, Win, OSX, linux) and a very active developer community. Combined with the Extra I/O board (MS3X) the box can do full sequential injection and ignition (With direct coil driving of ‘smart’ coils), launch control, traction control, ‘auto’ tuning in software, generic I/O of basically any function you can think of (including PID closed loop control), full logging via onboard SD slot and has a built in USB adaptor to boot!
In the next post I’ll go through the hardware and setup we used to make all this happen. I’ll also run through the ignition system that we switched over to ECU control.