Is TPS required in EFI? 1

[santorta]

Hey Guys,

i was just wondering about the basic Electronic Fuel Injection System for a Samll Single cylinder engine? If I use the MAP sensor for the load sensing and Crankshaft Signal for Speed sensing (Basically Speed density method), can i get away with TPS (& use a normal mech throttle body)? I came to know that TPS is only used for Acceleration Enrichment is those cases? Can't i sense the Acceleration with the changes in the MAP signal & inject the fuel accordingly? Can i do it without scarificing the engine response factor?

I know that there are too many questions, but those all are my thots running in my mind, May be you guys would have in-depth knowledge or work experience, which would answer my queries.

Thanks
Santo RTA

 

[Fabrico]

If it were carbureted, would the engine/application need an accelerator pump?

 

[TStaples]

A throttle position sensor is not a requirement to do a basic system. Unless you have some unusual requirement for particularly crisp throttle response (ie. maybe a go-cart motor), you can easily set up fuel injection on a small engine. I did a fuel injected briggs/stratton conversion motor in 1985 for an SAE supermileage program, with no TPS. I used to have a 1990 John Deere 285 tractor with a fuel injected two cylinder Kawasaki. It used a single twin-jet fuel injector spraying into a split intake, open loop, with no TPS. A speed density setup will work fairly well.

-Tony Staples

http://www.tscombustion.com

 

[santorta]

Hey TS,

Thankx for sharing for ur experience.

What happens if the air filter is clogged? How will the Speed-density system work, if the MAP is going to show erroneous value?

Santo RTA

 

[patprimmer]

I would suggest the appropriate action is clean the filter.

Regards

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

 

[TStaples]

Santorta,

patprimmer is right, of course! But, the engine has to be able to handle variations in air filtration efficiency because that is the "real world." If the air filter is heavily clogged, it will reduce the amount of air ingested at a given throttle opening and rpm. The MAP sensor will read this as a lower absolute manifold pressure, and reduce the fueling accordingly, assuming the engine is calibrated properly.

There are many speed density systems used on small engines. For example, Harley Davidson used an EFI system with a TPS up to 2001. When it switched over to the newer Delphi injection system in 2002, it switched to a speed density system with a MAP sensor. There are pros and cons to both types of systems.



-Tony Staples

http://www.tscombustion.com

 

[santorta]

Yeah right! I accept both of ur views. I was also thinking that if i could have a Barometric sensor and a MAP i will be able to find out the differential pressure across the throttle body (replacing the need for the TPS, from i will be able to get the information about the Altitude compensation and also i can include the diagonistic feature for the Air filter clogging identification.

But how quick the response of MAP system will be for a two wheeler engine (some quantification?) As you said before, it wont be as crisp as TPS system, but whether the MAP system will be atleast upto the mark, if not the best?

Santo RTA

 

[TStaples]

santorta,

I've been out of the loop in engine control systems for 3-4 years now. The technology continues to move forward. For a typical motorcycle application, I would investigate what you can find that has already been optimized for that type of application. I would think that ECU package size (Not a lot of available package space with adequate cooling, on a motorcycle) would be a bigger concern than the minor difference in throttle response (only maybe) that might be experienced between two systems. Talk to some companies that do custom hardware, and find out who is specializing in motorcycles.

-Tony Staples

http://www.tscombustion.com

 

[pmrobert]

I have real world experience with a rotary engine with ITBs and no TPS, using speed-density. Successful emulation of the TPS function by substituting MAP change rate (MAPdot) as the input works very well for me. Response is very crisp. The key to making this work involved both hardware and software manipulation. A smooth MAP signal is essential and can be realized by a combination of properly sized restrictors and volumes in the MAP vac line as well as software algorithms to further smooth the signal. Also, a dirty air filter looks just like a partially closed throttle to a speed density system - the MAP sensor doesn't know or care where the restriction is coming from. There's no imbalance in fueling; less air=less fuel. Alpha-N would be quite another story.

 

[jimwolf]

Using RPM/MAP without TPS will reduce throttle response as well as not allowing for fuel cuts on decel if needed. Any algorithm for transitional enrichment based on MAP deltas alone is reactive not preactive to the MAP change on TP opening. For good response the throttle opening RATE is used to determine additional fuel on transition more so than throttle position alone. Use a TPS if possible, it will be easier to solve the hardware installation issue than the drivability issues you may find later down the road.

 

[pmrobert]

jimwolf, I agree with everything you've said except the loss of decel fuel cut w/ MAPdot - it's very easy to implement. More inputs are better, no doubt, but in some applications (a small displacement, large intake, high revving engine) MAPdot is quite workable. TPSdot has it's own problems in the engine series I've just mentioned. The massive nonlinearity across the RPM range using TPSdot in this instance can be handled with software as well. There are several ways to skin a cat, depending on the cat!

 

[jimwolf]

Interesting, pmrobert, I have not seen a system that uses MAP rate for fuel cuts. You mention you use this with ITBs on rotary engines, what system is that you are working with that uses this in place of a TPS? I can't figure how they deal with the MAP Hysteresis at light loads/high RPM. I think we tried this for another reason once and found that under these conditions MAP can be within 1"Hg of idle MAP leaving little room for a cut threshold that doesn't false on light throttle. This of course wouldn't be much of a problem on a race engine, are you running this only for WOT type racing apps.?

 

[pmrobert]

My application is a Mazda 12A with a moderate street port used as a daily driver. I'm using a Megasquirt board w/ MS-II processor and MS2-Extra firmware. It's open source so you're not locked into a fixed system and make code changes fairly easily. Key points to getting MAPdot to work properly on a streetable application are several; a fairly clean MAP signal, proper setting of accel enrichment threshhold, proper setting of the VE table so decel cut works, etc. Clean MAP signal is attained by both mechanical volume/resistance in the MAP sensor supply line and software lag filtering algorithm parameters. Reading the MAP ADC signal at the same time in each engine cycle helps as well. I managed the mechanical filtering on mine by teeing both runners MAP signals to the common line leading to the MAP sensor, the (70cc? I don't recall exactly) volume in 7 feet of 3/8 cac line provides the "C" of the mechanical RC filter, I don't need a restriction. The MAPdot threshhold has to be set by trial and error, right on the edge of false triggering works well - this seems to be very important for rapid throttle response. There's a big difference between 50, 55 and 60 kPa/sec - 50 falses, 55 is great, 60 is laggy compared to 55. Decel is triggered when kPa < 30 or so and RPM > 1500. I idle and cruise at 36-40 kPa so there's no problem with false decel triggering. It has been a pain to get to work correctly as there was no easily available info on this concept when I started the implementation. It wasn't by choice - I would have preferred a TPS but am unable to mount one on my TB. It's been a lot of trial and error but I am pleased with the outcome. The nonlinear response of a TPS (for the small high revver with large intake case) has been avoided as well. I get 27mpg at 80mph cruise and 167 RWHP @ 7500 while running as cleanly and smoothly as my OEM fueled and ignited vehicles (Ford Ranger and Mazda RX-8). The 12A is currently undergoing surgery as I'm changing to coil on plug ignition.