[TZ350]

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Had the bike on the dyno today. Simulated the roll off power on again through the corner a few times at the sort of rpm you would be using on the track.

It showed real promise and actually looks like it is going to work. Mapping is a bit crude and the motor needs some maintenance. So I will strip the motor and spruce everything up then get onto mapping the fueling properly.

Anyway it looks very much like the Delta Crankcase pressure idea is the answer to keeping track of airflow through a two stroke motor.

When I have tidied up the software and commented it properly I will post a copy of the Pseudo MAP code.
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[PSIG]

[TZ350]

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I don't know how/where to post Arduino code so I have had to upload my software as a PDF.

Because your pressure sensor may change the timing of things a bit. The first step is to run the motor up on a carburetor and use a scope to view what the crankcase pressure signal and crank angle timing looks like and then if needs be the software can be adjusted to make its pressure measurements at a corrected crank angle.

The software takes a three high and low crankcase pressure readings. Selects the best of each, adds them to a rolling average and then determines the difference between the averages. The difference is multiplied by three and this value becomes a value that mimics a four stroke map value.

[TZ350]

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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.

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.
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[TZ350]

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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. 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 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.

I have attached a series of dyno runs recorded on my air cooled Suzuki GP125 engine at varying throttle positions.
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[DaveyB]

Hi TZ, bloody brilliant work mate.

Sorry, I haven't been keeping up to date with your project, but what you have achieved has been said by many "2 stroke gurus" to be impossible, due to the weird airflow through a two stroke engine. If you were over here, I wouldn't let you buy a beer for the day, perhaps two if you're lucky!

My project is making slow but steady progress, I have finally finished reconditioning the bottom end of the NSR250 and am prepping it for FI conversion. I have installed two barbs in the top part of the crankcase for the pressure transducers. I'm not sure if it really requires two, but I've installed them anyway? The engine has two distinct halves, so has basically a crankcase per cylinder. I was thinking of just using one half as the master and the other half as the slave, using the master's timings but delayed by 90 degrees (90 degree v twin). I'm still going to initially only control the fuelling, leaving the ignition and powervalve to the Zeeltronic ignition unit.

I have also installed an external air inlet into each of the A ports, controlled by one way valves. This is following on from your's and other's conclusions that at low revs, the exhaust gases are flowing freely back into the crankcases. The idea is by placing the air inlets within the A ports, as the piston descends it can also suck on fresh air and I'm hoping will stop most of the exhaust gases flowing down the transfers. As I mentioned earlier this has been done in the past. One guy did this to his KTM crosser, where he had to seriously lean out his pilot jet making the engine at low revs crisper and more responsive. I will in the future look at controlling the flow better with some air solenoids. But, for now the current method will be a direct feed from the sealed airbox. He was using a 1mm jet to control the flow, but said it wasn't really required as each time he made it bigger, the better it got. By the time the engine's pipe starts working the crankcase will be providing the majority of the air to the cylinder. So, as you've said even with a slight increase of pressure past BDC with the TPS reading 0%, the Speedy can be made to ignore the readings.

Clearly the high vs low pressure comparison works, but I can't get my head around only using 3 interrupts for taking the readings? For my engine, which uses crankcase reed valve induction and using the same cylinders as yours. The transfer port timing is symmetrical regardless of the revs, opening at 118 degrees ATDC (62 BBDC) and closing at 242 ATDC (62 ABDC or 118 BTDC). So shouldn't there be more interrupts around the opening/closing times?

[TZ350]

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KTM have managed it with the 2019 EFI 150cc model so high performance 2S EFI is on its way to being a normal thing.

https://transmoto.com.au/ktms-two-strok ... t-evolved/

They originally had "Flameout" issues when the pipe resonance changed but have since found a way to see changes in airflow. KTM aren't saying how but I suspect they are also looking at the difference between the High and Low crankcase pressure. The difference follows airflow, small difference low flow and the greater the pressure difference the greater the flow.

[TZ350]

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This is the software I developed for finding the difference between High and Low crankcase pressure.

1) It has one interrupt triggered by the ignition pickup.
2) From the time for the last cycle it works out the time in micro seconds to each analog read point. Six in total.
3) It takes three analog high pressure readings and three analog low pressure readings.
4) It selects the best of each three because the exact crank angle moves with rpm so you have to straddle the optimum area.
5) The best of each gets added to a moving average.
6) The Delta difference is averaged again.
7) The Delta is multiplied to get its magnitude to look something like a real MAP value.
8) The result is continuously output via a DAC card as a 0-5V pseudo map value until the end of the next cycle.

Currently the software looks at the TPS and if its over 45% then the output is standard atmospheric as read by the map sensor at startup. If it is less than 45% then the Delta pressure is used for the MAP value and if the TPS is 0% then the MAP value is forced to 0.2 bar.
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[TZ350]

I have installed an external air inlet into each of the A ports, controlled by one way valves. This is following on from your's and other's conclusions that at low revs, the exhaust gases are flowing freely back into the crankcases. The idea is by placing the air inlets within the A ports, as the piston descends it can also suck on fresh air and I'm hoping will stop most of the exhaust gases flowing down the transfers. As I mentioned earlier this has been done in the past. One guy did this to his KTM crosser, where he had to seriously lean out his pilot jet making the engine at low revs crisper and more responsive.

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I am very interested in this and look forward to the results.
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[TZ350]

Clearly the high vs low pressure comparison works, but I can't get my head around only using 3 interrupts for taking the readings? For my engine, which uses crankcase reed valve induction and using the same cylinders as yours. The transfer port timing is symmetrical regardless of the revs, opening at 118 degrees ATDC (62 BBDC) and closing at 242 ATDC (62 ABDC or 118 BTDC). So shouldn't there be more interrupts around the opening/closing times?

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The low crankcase pressure is just before TDC where the piston is still moving up and the incoming air has not had time to completely fill the crankcase. The high pressure is just before BDC when the piston catches up with air that has not yet been sucked up the transfers by the pipe. Gas inertia is everything here, port timing has only a small part in it.

The quality and speed of the pressure (MAP) sensor changes the timing of things so best to scope the sensors output at different RPM and loads then adjust the software's timing to suit. The three readings are so you can straddle the optimum crank angles for the best pressure readings.

I have attached a picture of the Speeduino EFI CPU which is in the box. The Arduino Nano and MCP4725 DAC, MAP co-processor are shown beside it.