Engine Torque Development

[gawnietheengineer]

Hello,

I have a question that regards the development of torque in a Spark Ignition gasoline fueled single cylinder engine.

The engine is required to drive an alternator directly off the crankshaft, as an experimental generator. The engine was not designed for this purpose but for a scooter. The engine speed is required to remain at a particular setpoint (2500rpm) and the average speed over a couple of cycles not to move substantially away from this setpoint. The speed is being controlled by setting throttle angle and reading the speed via a hall effect sensor and a PID controller. The engine was sized using attached which relates the break mean effective pressure (BMEP), torque and volume. The desired torque output was known for a desired power and speed, an average BMEP was taken to be 500kPa.

My question is: can various BMEP or even MEP in general, be developed at the same engine speed for the same engine, thus delivering more or less torque at that speed? What other factors affect this? Is throttle control suffice for this application?

If my post is unclear please ask me more questions.

Thanks

 

[ivymike]

can various BMEP or even MEP in general, be developed at the same engine speed for the same engine, thus delivering more or less torque at that speed?
Yes, definitely. That's exactly what's happening in your car when you are cruising the highway at a particular speed, and go up/down hills while using the accelerator to keep vehicle speed constant.

Is throttle control suffice for this application? Throttle control is how you would achieve changes in MEP for a spark-ignited naturally aspirated engine. Whether throttle alone will keep your speed variation within limits given (demand) load steps of a particular size is another question altogether. A big flywheel would probably help (oftentimes the alternator has high enough inertia).

 

[gruntguru]

The scooter engine may be designed to produce maximum BMEP (max torque) at higher rpm than your requirement of 2,500. It is often useful to have maximum torque fall at slightly lower rpm than the setpoint for control and thermal efficiency reasons. The elements affecting peak torque rpm are mainly valve timing and the length and diameter of gas flow passages. These elements can also be tuned to reduce peak torque more efficiently than throttling which causes increased intake pumping losses. LIVC (late inlet valve closing) is an efficient method of reducing output, particularly when combined with increased compression ratio.

For best fuel efficiency, the engine should operate at or below peak torque rpm and around 90% load. 90% load is difficult in genset applications because 10% headroom is usually insufficient for handling transients.

je suis charlie

 

[Tmoose]

The engines in consumer products like Snowblowers, lawnmowers, rototillers, generators and probably a bunch more run at essentially constant rpm regardless of load. A "governor" controls the throttle opening to maintain the set rpm. There are several types of governor on my devices, but all are mechanical. Whether the speed control is precise enough for your application is unknown.

 

[MikeHalloran]

If you can scrounge a mechanical constant-speed governor for your engine, you may not need the PID controller, depending on your speed tolerance. Not all engines are sold all ways, i.e., with/out a governor, with/out a mechanical governor, with/out an electronic governor.

With the PID, you will also need a throttle actuator to convert the PID output to throttle position.

Come to think of it, running an alternator at constant speed is, um, odd, because the big advantage of alternator based gensets is that they can be throttled back to a lower RPM when demand is low, and only need to run at max RPM when demand is high, or predicted to be high pretty soon.




Mike Halloran
Pembroke Pines, FL, USA

 

[MikeHalloran]

Of course, here I'm assuming that you're intending to use something like a car alternator, which actually generates AC but converts it internally to something like DC, which may then be inverted to AC if desired. Some small but expensive gensets run at variable engine speed to save fuel, and have internal inverters to produce AC at a frequency that does not correspond to the engine speed.

... as opposed to the generator heads sold for AC gensets, which are actually alternators and must run at constant speed in order to provide AC at the corresponding frequency.




Mike Halloran
Pembroke Pines, FL, USA

 

[gawnietheengineer]

The generator electrical load will be controlled independantly to be kept at 50 Hz with power electronics. All the engine has to do is run at a particular speed and develop enough torque at that speed.

From what I understand from gruntguru's answer the engine will produce the torque but it may not be at a high efficiency due to lower thermal efficiency and volumetric efficeincy.

I will consider adding a flywheel if the speed is not controllable. Has anyone any idea how I would calculate the inertia required to damp the speed oscillations? Or how to go about something like this?

 

[MikeHalloran]

Controlling frequency by varying the electrical load pretty much defines a dynamometer, not a generator. Maybe you need to review the original assignment.

Is this some kind of group project for school?



Mike Halloran
Pembroke Pines, FL, USA

 

[BrianPetersen]

What, exactly, is it that you are trying to do?

If you use an alternator - in which the AC that the alternator puts out is the frequency of the load - the frequency of the alternator output is locked to the rotation speed of the engine. It cannot be different. You cannot "keep it at 50 Hz with power electronics". The frequency is the rotation speed of the engine. It is "synchronous".

If that alternator/generator/?? is operating a stand-alone load (like a portable generator with a light bulb or whatever connected as a load) then the normal arrangement is to have the engine speed governed to a fixed (more or less) value and the output frequency will be synchronous with engine rotation speed. It will vary a little bit. But in most applications that are operated by portable generators, it really doesn't matter if they see 50 Hz or 53 Hz or 47 Hz as long as it is "somewhere near" 50 Hz.

If that synchronous alternator is being coupled to the line (a very large AC network) then its rotation speed will automatically be locked to line speed with no need to govern it. Any attempt by the alternator/generator to speed up or slow down will result in large currents being fed to or from the line which result in applied torques inside the alternator that bring it back into synchronisation. Of course, the utility power station's 500 MW power plant somewhere in the next county has to be governed to the correct speed, but it won't even know that your 2 kW generator is there. Varying the torque output of your engine (because you cannot change the speed) will only change the amount of current going to or from the network.

If you are using a NON synchronous machine - in which whatever current it generates is rectified to DC and then independently back to AC via an inverter - then yes, your generator speed need not be locked to line speed, but it also doesn't have to be very accurate, since it is electrically decoupled from the line.

Bear in mind that if this is a small engine that was designed for a motor scooter ... I have a 125cc motorcycle, and that engine idles at 1600 rpm, really isn't happy about running under load below about 6000 rpm, has peak torque around 8000 rpm and peak power near 10,000 rpm.