Injector size for 2 stroke dirt bike

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sparky
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Injector size for 2 stroke dirt bike

Post by sparky »

I am starting to source parts for my 250cc single cylinder EFI dirt bike project.

Single cylinder 250cc
I am expecting a HP at the rear wheel about 20HP
Max RPM is 8500
Max duty cycle - 90% (??)
Naturally aspirated
Fuel pressure from 43-50PSI

I have come across a couple of calculators that use max HP, duty cycle, RPM, fuel pressure. They calculate around 135 cc/min.

Another site suggests 190g/min for 200-300cc bike

Neither of these indicate 2 or 4 stroke, so I am not sure if these are valid or not.
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kb1gtt
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Re: Injector size for 2 stroke dirt bike

Post by kb1gtt »

Keep an eye on your electrical budget. Most bikes have a very small magneto or similar electrical generator. You may need to use LED tail lights, and similar energy reductions to have the ability to drive your fuel pump. The fuel pump consumes allot of energy.
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sparky
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Re: Injector size for 2 stroke dirt bike

Post by sparky »

Yep - all good with power.

Back to the original question: What about the injector sizing? Anyone have any experience there?
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kb1gtt
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Re: Injector size for 2 stroke dirt bike

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Can you post the calculator you found? It sounds like you found a very common set of equations for some very basic calculations, like those in the below calculator.
https://fuelinjectorclinic.com/flow-calculator

Did you find this one yet? If you view the source, it shows the back end calculations. It would be nice if someone updated this javascript. AKA it would be handy if it accepted both CC or CID. Basically pick a unit, and enter a value. Or even if they changed "Pulsewidth:" to "idle Pulsewidth:" those changes would make this calculator much nicer.
http://dainst.com/info/programs/pw_calc/pw_calc.html

I can think of many other adjustments which would be nice. Here are a couple improvements that could be made.
-- Peak Pulse width as well as minimum pulse width
-- Selecting or suggesting a BSFC. While a dyno is best, you can ball park things from the suggestions found here. https://www.raceworks.com.au/calculators/injector-hp-calculator/
-- Use rail pressure to determine flow, making this a mass calculator instead of flow calculator.

Any how, about your application. The dianst calculator suggests a 28.5 lb/hr (300cc/min) injector. It claims this injector at idle will be 1.35mS, and could produce a peak of 45 HP @ 80% duty, which is likely acceptable for both idle and full load.
300cc.PNG
300cc.PNG (30.01 KiB) Viewed 7007 times

The other calculator which suggests 12.85 lb/hr (135cc/min) is predicted to produce 1.78mS for idle, and a peak HP of 20. This would likely run lean at full load. So you probably want a slightly larger injector.
135cc.PNG
135cc.PNG (30.45 KiB) Viewed 7007 times
What I don't know is how well your engine will generate power. There will be variations in the amount of fuel that's required to get your 20HP. AKA high octane, vs low octane, vs E85, and many other factors will vary how much fuel you need to get that 20HP when under full load and full RPM. Of course 2 stroke tend to dump un-burn fuel in the exhaust. To compensate for this un-known amount of fuel that's simply dumped, you simply dump that much more with the injector. So in the above calculators, those predictions of BSFC are likely different than your particular engine.

A review of Brake Specific Fuel Consumption (BSFC) found at the below.
https://x-engineer.org/automotive-engineering/internal-combustion-engines/performance/brake-specific-fuel-consumption-bsfc/

This graph for one particular engine shows an engine's efficiency around 13.53% at idle RPM and idle loads. Then a peak of 33.85%.
https://www.x-engineer.org/wp-content/uploads/2017/08/Fuel-conversion-efficiency-table.png?f24857&f24857

Getting your BSFC from a dyno would be a much better method for determining if your injector is properly sized. With out knowing the BSFC, all your doing is making some wild guesses.
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sparky
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Re: Injector size for 2 stroke dirt bike

Post by sparky »

Thanks for the detailed info.

No access to Dyno, so I will have to guesstimate. Contrary to popular usage, I am not after peak HP, I am more interested in low end power and smooth delivery. So I am not worried too much about top end.

With the variety of calculators I keep getting aroun 130-140 cc/min. What is the drawback to picking too large of an injector - like a 180 or a 250cc/min (in this application)?

thanks!
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kb1gtt
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Re: Injector size for 2 stroke dirt bike

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If you pick to large of an injector, your idle pulses will be less than about 1mS. When this happens you loose precision in your fuel pulses, and it will be hard to deliver a proper amount of fuel. You really want your minimum fuel to be more than 1mS. For prediction purposes I would suggest staying above 1.5mS, as it allows some variation in the guess work. AKA if your guess is high by 0.5mS, and if your aim was for 1.5mS you are still OK when you try this in reality. However if you aimed for 1mS, and you guess is high by 0.5mS then in the real world it no worky and you need to choose another injector. Once you have your idle injector then it need to be able to provide enough fuel for the peak HP. This is commonly not at WOT, but is typically around 3kRPM and something like 80% TPS. If the injector runs lean under peak load, you can either try an injector which is closer to the 1mS or you can add a second injector which kicks in as a "staged" injector. AKA at peak loads a second injector is dumping more fuel.

In your situation, I would suggest you pick an injector with a minimum as high as possible, as well choose one which can provide more than 20HP. Find a happy medium for more HP, and increased minimum pulse width. After it's tuned, measure to see what happened in reality.

Do you have a fuel pressure regulator, which is using the manifold pressure to compensate for the difference in pressures in the manifold? AKA at WOT and low RPM, with a rail pressure of 50 PSI, the manifold side of the injector will be at something like 12 PSI, a delta across the injector of 38 PSI, which results in a certain amount of fuel being delivered at this pulse width of the injector. However if you are at say a high RPM, and you'd like 20% TPS, and your predicted pulse width is the same as it was in the last example, you manifold pressure will likely be something more like 5 PSI. The delta pressure across the injector in this situation is 50 PSI - 5 PSI = 45 PSI across the injector. This will deliver much more fuel than the same pulse width in the prior example. If you have a pressure regulator which adjust for manifold pressure, it maintains a constant delta pressure. This delta pressure is what allows the injector to deliver a properly metered amount of fuel.
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sparky
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Re: Injector size for 2 stroke dirt bike

Post by sparky »

Thanks for the info. I will go with 130g/min and then if need be, I will get a bigger one. In this size of system, the fuel pressure systems are pretty simple, so no adjustment for manifold pressure. And with 2 strokes manifold pressure is an erratic thing anyways.
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kb1gtt
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Re: Injector size for 2 stroke dirt bike

Post by kb1gtt »

If you do not have a fuel pressure regulator, you'll likely run lean on a regular basis. Running lean like this commonly results in carbon build up which causes hot spots, the hot spots cause erosion and eventually a hole in your head or piston. It is difficult to get longevity out of a lean running engine. I believe you'll have a MAP, so you'll known your manifold pressure. If you measure your fuel pressure, software could adjust for the lack of a regulator.

The short term erratic MAP is less concerning. A regulator reacts around 50mS, and has a tendency to smooth out the pressure in that period of time. This would allow your tuned fuel to be consistent. You may be lean for like 1 cycle, but you should have a proper AFR fairly quick, and should not have long term lean conditions.
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