Wednesday, 22 December 2010

Dual plane manifold

In parallel to the manufacture and testing of the Megasquirt controller, I have been working on the CFD analysis of options for the intake manifold. Having established my cylinder head model had at least a passing resemblance to reality in this post, I have moved on to creating a simple engine model comprising the manifold, two cylinder heads and outlet pipes representing the cylinders:

This model was then run 8 times, each with a different inlet valve being open. These were run with a constant pressure drop representing a high RPM running condition and maximum valve lift. This is the condition where runner mass flow variation will be most obvious. Once the simulations were complete, I interrogated the results and normalised them against the average mass flow for all 8 cylinders:

The chart indicates that the variation in inlet runner geometry produces quite a variation in mass flow into the different cylinders (from -11% to +15% from the average). This is not an issue with a carburettor providing the fuel because the mixing is done at the throttle plate so each cylinder gets an appropriate air fuel ratio. However, the fuel injection system will work on the total mass air flow being split evenly between the cylinders and fuel accordingly resulting in overly rich and overly lean cylinders. This is not conducive to a happy engine, so an alternative manifold is required. Onto the single plane design then...

Megasquirt Aux Board

I have now installed the output stage comprising the injector, fuel pump and Air Bypass Valve (ABV) drivers and this marks the completion of the base Megasquirt controller. However, the extras I am adding (Automotive connector, 2 coil pack wasted spark control, radiator fan control, tachometer output pulse, 2nd Exhaust Gas Oxygen sensor input, and 'engine running' signal) require an additional circuit board. I have tracked down what appears to be a relatively easy to set up and use PCB generation tool called Designspark. So far I have created the auxiliary board schematic:

Which then allowed me to create the PCB layout:

There are still a couple of details I need to sort out such as the flyback diodes for the relay driver and ABV, but I should be in a position to send this board for manufacture shortly. Before I do though, I would like to prototype some of the circuits just to make sure it will all work as planned!

Friday, 10 December 2010

Megasquirt Episode 3 - Revenge of the Sith

Progress has continued on the engine controller with the construction of the power stage with the required testing showing volts where they need to be and no volts where they shouldn't. Since I am not using the standard Megasquirt DB-37 connector, I am having to temporarily connect using discrete wires. Ultimately these will to be replaced with a flying lead to the auxiliary board containing the additional components for the wasted spark coil drive, tachometer output signal and the second exhaust gas oxygen (EGO) conditioning circuit.

The communication components are added next and checked for correct function thorough the programming connector.

With the board working according to the instructions, I was able to move onto the input conditioning circuits which take the engine sensor data and feed it into the main processor inputs. With these components in place, I could hook it up the the laptop and run the Megatune software used for communicating with, and tuning the ECU.

The engine sensors are replicated by the Stimulator so the pots can be twiddled and the corresponding reading on the real time display changes. So far so good...

Thursday, 2 December 2010

MegaStart - The Sequel

Following successful testing of the JimStim (well, making sure there was 12V where it should be and resistances change with twiddling the pots) I made a start on building the Megasquirt module proper. The comprehensive kit from DIY Autotune comes extremely well packaged and labelled, identifying each component, its value and important assembly information such as leg bend length and if the part needs to stand off from the PCB. I would heartily recommend this company to anyone considering the purchase of a kit, or indeed anything related. Anyway, I had to make a start on this:

First job was a modification (I can't do anything the conventional way!). I am planning to use wasted spark, PWM Idle valve control, ECU controlled cooling fan relay and an 'engine running' signal, so it seemed logical to have an auxiliary PCB to tidy everything up. In addition, I am not keen on soldering wires into connectors for automotive applications since there is a risk, albeit small, of wire breakage from fatigue. The standard solder bucket DB37 connector will have to be replaced with an automotive spec crimped terminal part. I managed to stumble across an enclosure of similar footprint, but taller to allow all of the additions to be made so I procured it from Maplin (part number N85AL) and set about scheming the design. Fortunately, the enclosure manufacturer kindly makes 3D CAD models available, so this was a relatively easy job:

With this complete, I could make a start on trying to turn it into reality. Since the enclosure is metric (100mm wide slot opening) and the Megasquirt PCB is imperial (4 inches), I had a small amount of filing to get the two to mate together. This had to be done with great care to ensure the intermediate copper layers (it is a 4 layer PCB) were not shorted to the case. All appeared to be well:

This has a knock-on effect to the heatsink strip that sits under the power devices. I had to shave a small amount off this as well to get the holes to line up:

With this all fitting together, I can make a start on stage 1 of the controller - the power supply.

Tuesday, 30 November 2010


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!

Friday, 26 November 2010

CFD Model Calibration

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.

Thursday, 25 November 2010

Intake Manifold - The beginning

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.

Watch this space...

Tuesday, 23 November 2010


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)!