Throttle Body Design

[santorta]

I am planning to fit a Electronic Fuel Injection for a Single cylinder four stroke Two wheeler engine. I am planning to remove the carburettor and design the throttle body with butterfly valve and fit a Injector near the inlet port. I think there is no necessity for me to design a throttle body with venturi profile. If at all, the venturi is need then fixing the MAP sensor, will lead to erroneous value.

Thanks in advance,
Santorta

 

[jbthiel]

There is no need to have a venturi with fuel injection. It is only needed on carburators to create a pressure differential, thus drawing fuel through a jet into the air stream.

Good luck on the project. A friend tried to put MegaSquirt onto a single cylinder road race bike with poor results. The biggest issue, he said, was getting a smooth vacuum signal. It had a tough time idling.

Cheers.

 

[patprimmer]

You can use the original carby with the fuel system removed and the venturi bored out to throttle plate bore size as a throttle body for fuel injection. It is not elegant, but is cheap, easy and uses a lot of stock components.

If you attach a side chamber to the manifold, and restrict the connection between it and the manifold runner and measure vacuum in that chamber you will get a more steady reading.

It could be tuned for increase response rate or improve stability by changing volume or restriction in connection

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[GregLocock]

Have a dig around your car engine. Observe where the pressure tap for MAP comes from. It is, wherever it is, for a reason.

Having said that, are you using MAP for acceleration, or just for cruise? If the latter then soem jiggery pokery with moving averages should sort it out. Acceleration mixture is usually controlled by a look up table based on throttle position, and its derivatives.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[santorta]

Thanks Jb, Pat & Greg.

Jb, Megasquirt offers a MAP sensor mounted on the PCB, and
this just measures the abosulte air pressure rather than the intake manifold vaccum. Are you talking about the version in which we need to mount the MAP sensor seperately?

I am planning to use only four sensors, namely TPS, MAP, Engine Coolant Temp and CPS. I am plannin to use MAP, both for fuel calculations and also for Cylinder Identification (instead of cam signal). So I should be able to sense the pressure drop during the intake stroke. I think the sensor location will only play a part in intensity of the signal, the farther i mount it i will get a weak depression of the signal, and the closer i mount it, the stronger the signal i get.

Pat, Thinking on your terms for reusing the carby, I dont think I should go for reboring too,I can use my existing throttle bore, since the design will match the existing flow calculations. Then I should consider having a additional flat passage after the throttle to regularise the pressure through out the passage till the inlet valve. And I should be mounting the sensor in the passage so that it captures the pressure values accurately.

Guys, Do I really have to have a IAC control. Can i do without it?

 

[Fabrico]

You probably could do without the IAC. MAP is only for fine tuning of the A/F ratio, you could probably do quite well witout that too.

Another thing that might work is change the MAP signal source to some kind of staged vacuum output, or staged throttle position.

If you're using intake pulse for CP information, you would need something better than a MAP sensor as they don't operate fast enough. Also, at a certain throttle opening, you would feel the fuel go on or off.

 

[alwieser]

I've seen throttle bodies with venturi's and noted that the MAP taps into this area for a vacuum signal. I've always assumed the venturi was used to amplify the vacuum signal for the MAP.

If this is the case, by using the original carb and keeping the venturi intact, you could tap into the passage from the venturi to the float bowl for your new MAP source.

Also, with the throttle blade offering a restriction to air flow, wouldn't reducing throttle bore after the blade help accomodate for this flow restrictin and possibly help smooth out the air flow?

 

[pmrobert]

Megasquirt measures MAP (Manifold Absolute Pressure). A separate MAP sensor is not required and will not change the reported MAP (on MS anyway). Speed/density EFI systems use engine speed(RPM) and intake charge density(corresponding to load)as the main inputs to an algorithm that calculates how much fuel to inject. There are several other inputs that can finetune things according to intake air temp, feedback from an O2 sensor, etc. You do not want a pressure tap in the venturi of an ex-carb (if that's what you're using for a TB) as it will report erroneously low pressures. GregLocock's advice is sound; you want a good,clean,accurate MAP signal and observing the method in which OEMs place them is a good learning tool with cheap tuition.