Engine Performance Maps

[NV7891]

Given the maximum power and torque ratings of a diesel engine, would it be possible to generate the throttle/Accelerator pedal map vs Torque at various RPMs? How can I construct this, or is it determined through tests?
Thank you!

 

[dgallup]

The vehicle manufacture will program in the response curve they want. It is not necessarily linear. Some of the old underpowered naturally aspirated engines would develop 75% of their torque in the first 25% of the pedal movement so that the driver would perceive them to be more responsive in around town driving. When you did floor them not much more happened. If the manufacture is trying to emphasize fuel economy they will program it the other way with smaller fuel delivery in the first part of pedal travel. If you don't have access to the delivery curve data then you will have to determine it empirically.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[ivymike]

There are a couple of popular ways to govern diesel engines: by requesting a speed, or by requesting a torque. For many truck engines with electronic controls the preferred method is to request speed (engine rpm) via the accelerator pedal, and the engine responds by delivering an amount of torque related to the difference between the requested speed and the current speed considering the performance limits of the engine. As such, you couldn't say that 25% "throttle" means 25% torque, since 25% throttle means a particular desired speed and the engine will change torque output to get there.

It gets a little bit more complicated than that even ... Oftentimes the (electronic) accelerator pedal provides a PWM signal to the engine, where having no pressure on the pedal gives a relatively low % duty cycle output, and having maximum pedal displacement gives a relatively high duty cycle output. Typically the lowest engine speed corresponds to about 20% duty cycle from the pedal, and the highest engine speed corresponds to about 80% duty cycle. In some implementations, calibration of the accelerator pedal is done automatically by the engine, such that initially the engine expects to see 25% duty cycle corresponding to low idle and 75% duty cycle corresponding to high idle, and it will re-map those duty cycle figures based on the pedal inputs it sees in practice over its operating life (typically expanding the range of operation when the duty cycle goes out of the previously established "hi" and "low" points). In these implementations there is no direct correlation between the pedal duty cycle value and a desired RPM. For example, it is not possible to input a 50% duty cycle value and expect the engine to operate at 1,800 rpm.

 

[ivymike]

If you had a truck with a mechanical governing system on the engine, where the pedal mechanically changed the fuel rack position on the engine to directly control the engine torque output, then you could probably come up with a map like the one you're asking about by calculating rack position as a function of pedal position and then relating that to your engine torque/speed map.

 

[gruntguru]

Governing of automotive diesels is usually a combination of speed and torque reduction as throttle is reduced. If you take the full load torque curve and shrink it towards the origin, you will have the part load curves.

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