Fuel Injection

[baldilocks]

Before we start I am not posting this for any other reason than to understand how the fuel injection works, its not criticism and its not praise, I'm just interested.

In the starting issues thread it was mentioned that the Ossa like a lot of two strokes needs to be revved from time to time to clear it out and this got me thinking.

In a carb fuelled two stroke we have an inlet port and transfer ports. My limited understanding is that as the piston travels up this creates a vaccuum which sucks fuel mix into the cylinder below teh piston. When the piston returns this mix is forced out via the transfer ports and ends up on top of the piston. The problem with this is that some of the mix escapes at this point without ever being burnt.

Does this unburnt fuel, which ends up in the exhaust, clog the system and this is why a two stroke needs to be cleaned out or revved from time to time ?

My next question is why this is in the Ossa forum.

On the Ossa do you still have an inlet port and transfer ports or does the fuel just get injected onto the piston surface ? It seems with fuel injection it would be more efficient to only have an exhaust port in terms of wear on the piston and emmissions. But there may be many more difficulties with this than I am aware of.

I'm think from memory Honda had such a design years ago and took the bike across one of the poles, it was reported in MCN. As the fuel was directed toward the cylinder head hardly any unburnt fuel escaped and therefore emissions were dramatically reduced. Obviously if it was that good in terms of performance it would be in mass production by now so there must be more to it ?

[gasgas249uk]

What an interesting topic. My understanding of why a two stroke bike needs to be revved out from time to time is the following

Its my understanding that Its all down to the oil that we dont burn off.

2 strokes generally duct fuel/air/oil mixture into and through the bottom end ,which circulates around the crank. The fuel mixtures heavier part is the oil and gravity pulls this from the cylinder walls down wards.

Fuel air mixtures are traditionally set rich from the factory as this increases reliability and reduced heat and the associated seizures. This setting also increases the thick oily mixture that gathers in the bottom end.

Thick oil/petrol mixture builds up in the crank case reducing the efficiency for the fuel circulation in the engine - hence the reason why a partially choked engine (as described above) wont rev cleanly untill cleared. There is only a tiny amount of clearance around the crank and 100ml of thick oily mixture will effect the crank turning.

When you clean the bike out by revving it, the greater forces and suction created drag the oily excess mixture into the cylinder burning it , hence the smoke for a short while.

Bearings,pistons and cylinders only needs the thinnest of coatings of oil. Often bikes are set up to give them a lot more causing the problem you describe.

I dont know for sure , but im guessing the Ossa FI will be traditionally inducted via the crank.

This allows mixture to lubricate the mains , crank and cylinder. Its just an effective way of circulating the mixture on time , every time. Inducting to the top end ,would require a separate bottom end lubrication system.

[feetupfun]

baldilocks the system OSSA uses injects the fuel in the linet tract. It is called throttle body injection. Same principle as on a Honda Montesa 4RT. Some modern cars and modern outboard motors have direct injection of petrol, where the injector sprays petrol into the combustion chamber.

About why the OSSA still have an inlet port and transfer ports, is because that is how the air and fuel gets to the combustion chamber. Even if it had direct injection, the inlet port and transfer ports are needed to get the air in.

[copemech]

"Before we start I am not posting this for any other reason than to understand how the fuel injection works, its not criticism and its not praise, I'm just interested."

It works like a standard bike, yet a precise amount of fuel is sprayed in as compared to a carb in which it is hopefully sucked in in the correct proportion.

This amount is adjusted by the open time of the injector, and is able to base the quantity more precisely dependant upon RPM, throttle opening and engine load. It will also compensate for base changes in barometric pressure and temps if all the basic inputs to the controller are there. The mapping based upon these inputs can be tweaked, as well as ignition timing depending upon the system.(yet these may be basic)

As far as revving them out on occasion, yes they will likely build up excess oil in engine and exhaust just like any other(trials) bike. I think I have seen that Ossa recommends mix ratios that are quite lean by most standards, 120:1 comes to mind here.

Not sure just how much that in itself has to do with the fuel injection itself, yet I do believe that for the most part many if not most of us currently run far to much oil for normal putting around and average section use and it all accumulates. One needs revs in the motor and heat in the exhaust(load) to rid of this.

This would be different on road trials or mid range and high rpm running, Yet just how many do that?

Edited June 1, 2012 by copemech

[billyt]

What an interesting topic. My understanding of why a two stroke bike needs to be revved out from time to time is the following

Its my understanding that Its all down to the oil that we dont burn off.

2 strokes generally duct fuel/air/oil mixture into and through the bottom end ,which circulates around the crank. The fuel mixtures heavier part is the oil and gravity pulls this from the cylinder walls down wards.

Fuel air mixtures are traditionally set rich from the factory as this increases reliability and reduced heat and the associated seizures. This setting also increases the thick oily mixture that gathers in the bottom end.

Thick oil/petrol mixture builds up in the crank case reducing the efficiency for the fuel circulation in the engine - hence the reason why a partially choked engine (as described above) wont rev cleanly untill cleared. There is only a tiny amount of clearance around the crank and 100ml of thick oily mixture will effect the crank turning.

When you clean the bike out by revving it, the greater forces and suction created drag the oily excess mixture into the cylinder burning it , hence the smoke for a short while.

Bearings,pistons and cylinders only needs the thinnest of coatings of oil. Often bikes are set up to give them a lot more causing the problem you describe.

I dont know for sure , but im guessing the Ossa FI will be traditionally inducted via the crank.

This allows mixture to lubricate the mains , crank and cylinder. Its just an effective way of circulating the mixture on time , every time. Inducting to the top end ,would require a separate bottom end lubrication system.

Gasgas249

I think you penned the reply eloquently. I agree with oil been sucked up from the crankcases due to separation etc.

However may I offer the fact that revving a bike does not produce a higher vacuum as the strength (torr or Manometric Units) of the vacuum is set by the stroke and piston size, cavity to fill and atmospheric pressure and is a constant.

Revving a bike increases the frequency of vacuum pulses per minute therefore simulating a higher vacuum effect i.e. a hundred RPM equates to say 100 vacuum pulses per minute vs 1000 RPM equating to 1000 vacuum pulses per minute. It is this more frequent pulse/pull that draws up the oil from the crankcase. A 125cc does not become a 300cc just because you revved the engine up.

The second point is that the oil in the gas oil mixture does not actually burn but rather gets spat out the exhaust.

Edited June 1, 2012 by BillyT

[borus]

main crank bearings are lubricated by the crank oil, mixture is 110:1 , case injection with the injector located between the reed block / throttle body and cylinder, sensors are barometric pressure, air inlet temperature, coolant temperature, throttle position.

[deonattard]

Borus do you know if there is any fail-safe functions in the ECU? Ie. Say your water pump as failed, bike is over-heating, will power be reduced or will the bike turn off - or will it just blow up like a normal non-electronic bike?

[copemech]

The second point is that the oil in the gas oil mixture does not actually burn but rather gets spat out the exhaust.

Of course it BURNS you muppet! What do you think makes SMOKE! Now this does require some heat in the exhaust as the oil migrates through the motor, and low heat burns less under low load conditions!

Edited June 2, 2012 by copemech

[billyt]

Miss Piggy

What, no wise crack about my vacuum comment?

Perhaps "burned off" is a miss nomer.

The oil changes state in the ignition process.

What do you think smoke is? Co2 basically? Smoke is still carbon based. The oil does not just vaporize.

Some gets blown out and some changes state. Oil is carbon based, smoke is carbon based.

The oil in the two stroke is to lubricate not produce energy. The bike would run on just gas alone not for long but it would run before it seizes up.

"By combusting hydrocarbons within the fossil fuels, carbon dioxide (CO₂) gas is liberated (as well as many other compounds such as CO which result from imperfect combustion). This is also the greatest reason hydrocarbon fuels lose much of its weight after completely burning, the carbon content within the hydrocarbons have been lost from burning and conversion into CO₂."

Read more: http://wiki.answers....e#ixzz1wbvc7FbM

Back to you Miss Piggy

Edited June 2, 2012 by BillyT

[tomch]

Borus do you know if there is any fail-safe functions in the ECU? Ie. Say your water pump as failed, bike is over-heating, will power be reduced or will the bike turn off - or will it just blow up like a normal non-electronic bike?

I had the temp sensor for the fan fail and then it boiled dry, needed to replace the o rings and the fuel injector head that was made of plastic.... It didn't cut out until I lost compression thru the o rings.....

[borus]

Borus do you know if there is any fail-safe functions in the ECU? Ie. Say your water pump as failed, bike is over-heating, will power be reduced or will the bike turn off - or will it just blow up like a normal non-electronic bike?

I don't believe there is...

[copemech]

Miss Piggy

What, no wise crack about my vacuum comment?

Back to you Miss Piggy

Oh yes, I forgot that, YOU SUCK!

Now back to other things that do not pertain to Billy blowing smoke up my ass!

It would not be the vacume within a 2T motor that purges excess oil form the crankcase that accumulates at low RPM runnings of a typical trials bike, but the velocity of the incoming air at higher revs and engine loads that start getting things flowing and burning off. Actual crankcase vacumes would be decreased(pressure increased) at larger throttle openings allowing more air to flow in increasing velocity.

Once the oil is pitched up top,it requires heat and flow of the outgoing mix into the exhaust to really light things off and keep the drooling oil from accumulating there in the muff and stuff! Even the accumulated oil residues there will start flowing a bit with some heat to get things going! At least spat out the rear!

If what I think Borus said about the mains being lubricated by gearbox oil on the Ossa, then the higher oil ratios at 110:1 is easily achieved with modern synthetics. Fact is most punters on conventional bikes could easily go to 100:1 without issue. Seems few take heed of anythis, they just grunge things up and bitch when the things are clogged!