So one of the aims of my project is to use in-cylinder pressure as a means of providing a measurement of what is going on with the engine, and hence use this to in some way control my ignition and fuelling.
I needed a high speed ADC for sampling in cylinder pressure for my ECU, and settled on trying a few from Texas Instruments. I am using an optical encoder on the engine which provides 720 pulses per revolution . If I took a sample every pulse, then at 6000 RPM (100 Hz) then I would be looking at about 720 * 100 Hz = 72,000 Samples per Second (SPS) or 72kHz ( although I realise now that I could take a sample on each edge, resulting in 1440 samples per revolution, or 144,000 SPS). That’s pretty high speed, considering most ADCs built into microcontrollers take a few microseconds to perform their conversion which results in a sampling rate of perhaps up to 50,000 SPS max for the Arduino for example. DSP chips or higher end micros probably have better performance, but for the sake of learning, I fancied trying to use an external ADC anyway.
Just a quick one on what I’ve been up to the last few days. I now have the engine set up inside so that I don’t have to keep pushing it outside or waiting for the rain to stop. I’ve also set up an optical encoder, in-cylinder pressure sensor, and AVL IndiSet high speed data acquisition unit to capture data on a 0.25 degree crank angle resolution.
More progress made today, I connected up the alternator to the engine to begin testing it under load. This was pretty successful, and the ECU all performed as expected, with only a few tweaks to the PID controller parameters to improve the AFR control. I restricted the range of fuelling down to as restricted as possible to prevent the system setting wildly large or small fuelling amounts under certain conditions. I finally managed to get the PID to maintain the AFR slightly rich within a few percent under steady state conditions.
I have now managed to run the engine with electronic fuel injection (EFI) and electronic ignition, both controlled from my ECU. I’ve also managed to run in closed loop lambda control to maintain the correct air to fuel ratio.
When testing engines, some method to ‘load’ it is required. If the engine crank shaft is not connected to anything, then the engine will just be able to run up to maximum speed very quickly, but will not be producing much torque. If you want to begin testing how the engine performs if you actually want to do some work with it, then you need a way of making the engine produce torque and power.
Success! I have started and run the engine on my own electronic ignition. The problem was in the end quite simple, it turns out that the missing tooth wheel on the crank was aligned differently to what I expected. I thought that the missing tooth occurred 240 degrees after TDC, but in fact it appears to be aligned almost exactly at TDC. This meant that my spark was happening around Bottom Dead Centre, which is no use at all!
Having thought I’d almost nailed my ignition timing problems, I tried testing at higher engine speeds (up to 6000 RPM) only to find that the ignition timing began to become erratic again.
From my couple of brief tests, I’ve identified a few problems which are preventing the engine from starting up and running for more than a few seconds.
- The ignition timing at very low speeds < 500 RPM is very erratic, and not at all on time.
- Under heavy acceleration the ignition timing is erratic, for example when pull starting the engine
- There are some occasional misfires
- The ignition timing is generally not massively stable – sometimes varying as much as 10 degrees(!) Continue reading
I took some pictures today of the set up I’m using to develop my ECU, the engine is a single cylinder 2.5 Hp Briggs and Stratton engine. It’s mounted on a trolley with the circuitry required to drive each of the components of the engine there’s more pictures available here: http://www.flickr.com/photos/gadgit/sets/72157633929512218/