With the arrival of my fuel injection controller kit (Megasquirt), I have turned my attention to my soldering iron. The first task is to build the 'stimulator' which is a unit that replicates the engine signals allowing you to test the Megasquirt ECU during and after its construction. There are two readily available options - the Megastim from the people who designed the ECU, and the one I opted for - the JimStim v1.5. The advantages for me are that the JimStim includes a chip that simulates the trigger wheel output I will be using (36-1 teeth) as well as a breakout block to allow the real sensors and signals to be wired into the system. This means I can test the ECU purely on the Simulator and then introduce the real engine hardware one piece at a time. Fault finding should be a little easier (I hope!). Before all that can happen, though I needed to turn this:
With a flurry of flux smoke and singed fingers, into this:
Now if only there were some kind of device I could use to test the stimulator, all would be well!
In order to have any confidence in the CFD analysis I will be undertaking, I need to check that the model has some bearing on reality. I have the flow bench data for the cylinder head I am intending to use so I want to know how well my CAD model matches up with the empirical information. To achieve this, I have created a 3D model of the cylinder head and set it up in a virtual flow bench i.e. I added a radiused inlet port and a pipe to represent the cylinder.
Then it was a simple task of adding the pressure drop according to the real flow numbers I have and running the simulation for a number of different valve lift values. The results are as follows:
As you can see, the predicted flow numbers correlate reasonably well with the measured values. There is around 7% difference in the region of interest, which considering the CAD model has been cobbled together from photos and limited dimensional data available on the web, I think is none too shabby. I will only be using the model to compare different options rather than looking at absolute values, so I believe this is a reasonable starting point.
The aftermarket fuel injection controller I will be using (Megasquirt II) is typically set up to be a speed-density system meaning the computer measures the engine speed and manifold pressure, then calculates the mass air flow into the cylinders. The injected fuel is then a proportion of that air mass to achieve the desired air : fuel ratio (nominally 14.7:1, but varies with operating conditions). Closing the loop with a lambda sensor in the exhaust stream confirms the desired ratio is achieved and if not, some 'live' tweaking can occur. This system works well, but is based on the assumption that the total air flow is evenly split between the cylinders. This is where the intake manifold comes in, as it is critical to splitting and delivering the air charge to the individual cylinders. There are a couple of options for a fuel injected manifold on the Ford 302; use a late model stock system or modify an standard carburettor manifold with injector 'bungs'. The appearance of the stock system is not really in-keeping with the look I want for the engine bay and I have concerns over how well the air will be distributed with the carb manifold (either single or dual plane). To allay (or confirm!) those concerns, I have decided to do some investigation work which involves some engineering analysis. Fortunately, I have access to a CAD workstation with CFD (Computational Fluid Dynamics) software and I will be looking at comparing the performance of the options for my intake manifold.
Welcome to my small corner of the Internet. In this blog, I am planning to record the trials and tribulations of the construction of my Cobra replica kit car. As I type, I am planning to build a Gardner Douglas Mk4 with a Ford 302 Small Block V8, though nothing is set in stone until it is paid for! I have what some might consider ambitious plans for the specification in that I am hoping to manufacture a large number of the parts myself including the fuel injection system, heating controls and I am currently exploring the possibility of custom dashboard instruments. This is mostly driven by the fact that I have more time than money and can spend one to save the other, but still end up with a professional looking product (I hope)!