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Successfully fuel injecting a single cylinder high performance two stroke, i.e., something making more than 10 bar BMEP and greater the 10,000 RPM has been very difficult until now. Because it was impossible to apply four stroke air flow measurement techniques like MAF, manifold mass air flow and MAP, manifold absolute pressure to a two stroke.
A two strokes inlet manifold and crankcase have entirely different pressure profiles to a four stroke. Whereas a four strokes manifold pressure decreases when the throttle is closed and is at its highest when at WOT, wide open throttle with a range of 0.2 bar to 1 bar on a normally aspirated engine. A two stroke is the other way around. Maximum manifold and average crankcase pressure at minimum throttle and reduced pressure at WOT, wide open throttle and the range is 1 bar at no throttle to about 0.7 bar at WOT, wide open throttle.
The reason for the two strokes pressure profile is because at WOT and in the power band, the pipe is sucking hard on the crankcase and so reducing the pressure there. At closed throttle there is little or no pipe suction and atmospheric pressure floods back down the stinger and up the expansion chamber flooding the crankcase to atmospheric pressure with exhaust smog.
A two stroke can idle with a crankcase full of smog because a small amount of fresh mixture randomly finds its way through this soup to the cylinder. At idle a two strokes cylinder is filled with random proportions of smog and fresh mixture each cycle. So you have to run a bit rich here to give a fuel molecule a better chance of finding a O2 to combine with.
It is very easy to setup a fuel injected two stroke for maximum power on WOT wide open throttle as air flow there is consistent and the Alpha-N fueling topology works well there. Alpha-N is TPS throttle position vis RPM and relies on consistent airflow.
But accurate fueling becomes increasingly difficult at lower throttle settings because the suction action of the pipe becomes increasingly variable and changes in air flow less consistent or predictable at lower throttle settings. At lower throttle settings the VE, volumetric efficiency topology is more appropriate. VE, volumetric efficiency is MAP, manifold absolute pressure vis RPM and is suited to variable airflow.
In a two stroke variations in air flow through through the motor can be seen by watching the Delta crankcase pressure. The difference between the lowest crankcase pressure near TDC and the highest near BDC. This delta is a reliable indication of changes in air flow. The greater the delta the greater the air flow.
The theoretically correct crank angle for the highest pressure is around 160 ATD and lowest about 20 BTDC but the quality and response of your pressure sensor may change things.
The first step is to see how your sensor responds. Maybe run the engine up with a carburetor and view the sensors output and timing on a scope.
This is how I would setup the EFI fueling using the delta crankcase pressure concept.
The Red area below 45% TPS and in the RPM area where the pipe is working I would setup the fueling for VE volumetric efficiency. MAP vis RPM.
Every where else in the Green area I would setup fueling as Alpha-N. TPS vis RPM. Airflow is consistently predictable here.
And in the Purple area below the RPM where the pipe works and TPS = 0% the MAP value needs to be set at 0.2 - 0.4 bar. This is necessary because when the pipe is not sucking strongly the crankcase pressure rises to someplace close to 0.8 - 0.9 bar of dirty air. This happens because the pipe is not drawing airflow through the motor and atmospheric pressure and stale exhaust gasses flow back into the crankcase raising the pressure there and in the inlet tract.
The motor can idle with a crankcase full of dirty air because some fresh air/fuel makes its way through the smog and randomly finds its way into the cylinder. At idle the cylinder has some fresh air and a lot of smog.
A copy of my PesudoMAP software in PDF format for an Arduino Nano.
Basically this determines the Delta between high and low crankcase pressures and outputs it as a pseudo MAP value that can be used by any EFI system that can use MAP as an indication of airflow.
With their latest 150cc TPI offering KTM have shown that high performance two stroke EFI is possible. They use a crankcase pressure sensor but aren't about to reveal how seeing changes in air flow is done. But with the Delta crankcase pressure reading concept I have revealed the secret of how to see the all important changes in airflow through a two stroke motor.
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