Limiting max engine load

[Piwoslaw]

I've been thinking: If a car has a fly-by-wire gas pedal, then could the signal be modified to limit max engine load? Here's the idea: If you press the gas pedal until you feel a resistance, then the TPS signal will be constantly modified to keep the engine load at the set maximum, for example 80%. Press the pedal harder and you will have full load.
I've seen the idea of a spring under the gas pedal here at EM, but limiting its position doesn't always translate directly into limiting engine load. Having something keep the load at 80% is much easier than glancing at the SG or UG every second.
The question is: How do OBD gadgets compute engine load?

[Daox]

Load should mainly be based upon manifold vacuum, but also engine rpm. Most cars these days don't have MAP sensors though, so they use the data from the MAF sensor. Using the MAF sensor you need to know RPM.

[capnbass91]

With an Ultra-Gauge the manual says this:. Where engine load is calculated as:
% Load = Current Air Flow/Peak Air Flow. OR % Engine Load = Current Engine Torque/Peak Engine Torque. Method used if vehicle dependent.

80% Engine load is best for getting the most power out of the engine? Even flooring it I can barely get it to 60% in my car.

[AXMonster]

You can have a special engine calibration (remap) made with a driver demand map (also called driver's wish in some software) to be flat mapped at one region of travel. For example 40 to 80% travel could demand the same fuel level, then it could ramp up to full fuelling from 80% to 100%.

I have the driver demand map for my car and may well do this to see how it works! Thanks

[Piwoslaw]

Originally Posted by capnbass91

80% Engine load is best for getting the most power out of the engine? Even flooring it I can barely get it to 60% in my car.

I mentioned 80% since that load usually gives best BSFC (depends on rpms, too). Someone (Euromodder, I believe) once wrote that he can see 99% load in his SG when the pedal is pressed only 1/4 of the way, after that only the only thing that increases is the amount of fuel wasted in the engine.

[Steaming]

I'm fairly new to all this, but If I mash the pedal in my v8 enough to cause SG to read 80% engine load, I expect to waste fuel burning rubber. I'm assuming the 80% figure is relative to smaller engines? lol

[Daox]

It is not just limited to small engine. But, it does go to show how oversized your engine actually is. Unfortunately for you, its just not practical to use the engine in its most efficient range.

[user removed]

I always thought of 80% load as based on RPM, not just the peak torque at the highest reading. Few high powered cars will spin tires at 80% load, in higher gears, at 1600RPM, probably none I have ever driven and that covers hundreds, including a De Tomaso Pantera, 390 Mustang GT, 383 Formula S Barracuda, etc.

If 80% load is a reading of about 2 inches of manifold vacuum then you can easily achieve that reading at low engine RPM. Maybe I am wrong, but the peak torque reading when on a dyno would be at precisely the same RPM that would produce the most power for the least fuel. The problem is that point is not really practical for any sustained period of time.

Within the practical operating range of most engines if you put precisely the correct (and constantly increasing as engine speed increases) load, then you can run that engine at 80% load from say 1200 to 6000 RPM by controlling the load through the dyno.

Seriously overpowered vehicles can still perform economically as long as they are not designed for such high power levels at high RPM that they suffer serious efficiency problems at low RPM. Even a NASCAR engine can get 6 MPG at 180 MPH, because that is what it is designed to do. Variable valve timing, fuel injection and feedback mixture control systems make it much more practical versus cars like the late 60s BOSS 429 Mustang that would barely idle at 1300 RPM. Today with variable valve timing and fuel injection, versus a toilet delivering fuel, the same engine could be made much more practical for everyday use.

regards
Mech

[dcb]

A couple friendly points to clarify

Originally Posted by Old Mechanic

...Maybe I am wrong, but the peak torque reading when on a dyno would be at precisely the same RPM that would produce the most power for the least fuel.

It isn't precisely the same RPM, I have seen some charts where peak bsfc is a thousand or so rpm off of peak torque.

Originally Posted by Old Mechanic

...Seriously overpowered vehicles can still perform economically...

But for simple commuting power needs, the seriously overpowered engine spends a lot of energy fighting with itself, thus will always be at an efficiency disadvantage to a "right sized" engine. And of course power corrupts

[E4ODnut]

[QUOTE=capnbass91;274492]With an Ultra-Gauge the manual says this:. Where engine load is calculated as:
% Load = Current Air Flow/Peak Air Flow. OR % Engine Load = Current Engine Torque/Peak Engine Torque. Method used if vehicle dependent.

I don't mean to hijack this thread, but I've been curious as to how to calculate engine load. The above quote makes sense to me.

If you use torque, then you have to have a means of determining torque. A dyno is the only thing I can think of. Not practical for most people.

Air flow is probably the most practical, but it depends on how you can determine air flow.

If you have a Mass Air Sensor equipped vehicle, and the means to interperet it's signal, the MAF sensor has done the work for you and the math is simple, no other inputs needed.

However. What if you don't have a MAF sensor and use a speed density system to calculate air flow (Manifold Absolute Pressure) sensor, RPM and engine displacement? The same could be said for Alpha - N, in which you don't use MAP, but rather throttle position, RPM and engine displacement. Of these two methods, it is generally considered that MAP/RPM gives a more accurate picture of actual engine load.

Now the calculations as to what your air flow actually is get somewhat complicated. With both methods you also have to take into consideration the volumetric efficiency of the engine at different speeds and loads. If you know the volumetric efficiencies, then you can do the calculations but these numbers are not normally available. Other than general statements to the effect that volumetric efficiency normally tracks the torque curve, I haven't come across any published figures for any engine.

So, my question is, how can you determine percent of engine load, with any reasonable degree of accuracy using a speed density or Alpha - N system?