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A question about fuel injection…

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Posted by: Kav

I’m making a new thread so as not as to hijack a different one.

Quote:
Originally Posted by Rich Rohrich
You'd need to run a speed density system based on manifold absolute not mass air flow. None of the current street bike systems will be even close to workable but the F.A.S.T ECU could be adapted without too much work. Kinsler can supply a single barrel throttle body and RC Engineering can help with sizing a custom injector .


Why can’t we run a mass airflow style fuel injection system on a dirt bike? Is it due to sensors that are not up to the task (yet)? Or is there some other reason?

I can’t really see where the design of a two-stroke or the four-stroke engine could not be fuel injected via a closed loop mass airflow system.

I would figure that a four-stroke set-up would be very similar to what we have in a modern car. I.E. a MAF sensor located in the intake track before the head, and an oxy sensor in the header pipe.

As far as a two-stroke would go, you could have the MAF sensor placed right before the reed valves in the intake track. Assuming that you don’t have a crake case leak you would be able to accurately measure the amount of air going into the crankcase. Then by measuring the amount of air that goes into the crankcase the next time. You could determine the amount of air that went into the cylinder from the crankcase. An oxy sensor in the expansion chamber could then fine tune the system.

I’m going to guess that a leaded fuel would be a bad idea for this

Then again it’s really late/early, and I could be totally wrong.




Posted by: cr250can

The problem I see with a MAF system on a bike is 2-fold.

1- the way the MAF sensor works is by a heated element that has electricity going through it all the time. as the engine draws more air past the sensor the element cools and this changes the resistance of the element; and as we know as resistance changes in a circuit so does the voltage. The computer then reads the change it adjusts the fuel. This element is thin and this could cause problems becuase a dirt bike gets banged around alot causing a failure of the element. also it would not work well in a 2-stroke especially if there was a chance that the fuel oil mixture will come in contact with the sensor.

2- A MAF system does not deal well with air leaks. and as we all know air leaks are way to common on dirtbikes (especially 2-strokes)

One note I might add, the O2 sensor on any fuel injection is not a primary input to the computer. It is used only as a "fuel trim" to regulate the fuel mixture for cleanest emissions.


Posted by: Rich Rohrich

One of the biggest weaknesses of MAF systems is they can't tell the difference between air moving forward or backward. As a result when you get large flow reversals in the intake tract, air can flow backwards over the MAF element and make it think there is a greater mass of air than really exists. Any isolated runner system with a large overlap cam would be subject to these types of issues, and an engine with a super short intake tract like the CRF450 which runs over a very wide rpm range would have especially large issues with this.

By using a speed denisity and looking at rpm, inlet air temp, load (MAP or TPS) , coolant temp, and exhaust oxygen content you can tailor a fuel and ignition map in an ECU very accurately regardless of the influence of cam and intake resonances.


Posted by: cr250can

Thanks rich I forgot about that. Automobiles rarely run at open throttle and have long intake tracts. So I didn't even consider that.

I like speed density better anyway.


Posted by: bikepilot

I am in agreement with everyting posted here, but would like to point out that there are many different types of mass airflow sensors. For example, Toyota used a mechanical sensor on most of their late 1980's-mid 1990's vehicles. It basically consists of a small flapper in the intake. As air flows through it pushes the flapper open, based on the posistion of the flapper the computer knows how much air is flowing into the motor. Of course such a sensor is not necessary for a fuel injection system - just look at all the fuel injected street bikes, none that I know of use a mass air flow sensor. Plus its so much easier and more percise to re-map a computer than to jet a carb

I love being able to plug my laptop into my TL1000 to adjust the fuel and ignition mapping:moto


Posted by: Faded

Quote:
Originally Posted by Kav
As far as a two-stroke would go...An oxy sensor in the expansion chamber could then fine tune the system.

If you place an 02 sensor on the expansion chamber you will thoroughly confound the ECU by the readings it gives. As exhaust gas is scavenged from the cylinder part of the next intake charge follows behind only to be shoved back into the cylinder by the reflection of (some) of the exhaust. The sensor would be reading these 'cycles' confusing the engine management system.


Posted by: Studboy

Some snowmobiles have a temp. sensor in the expansion chamber, but it changes the ignition timing curve, not the fuel curve.



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