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Difference between constant speed and variable speed engines

Difference between constant speed and variable speed engines

(OP)
Hello everyone,


I wanted to what exactly is the difference between the constant speed and the variable speed engines?
I am inclined mainly on Diesel engines.

RE: Difference between constant speed and variable speed engines

The throttle position.

RE: Difference between constant speed and variable speed engines

(OP)
Thank you...
But why exactly is it not used in constant speed and used in variable speed engines?
Also, why throttle is not used in diesel engines?

RE: Difference between constant speed and variable speed engines

Most constant speed engines are running generators where it is very important to keep the speed constant to produce the desired 60 or 50 Hz AC waveform. These engines can have everything optimized to run at this single speed which will improve efficiency. Variable speed engines are usually prime movers in things like trucks, buses, tractors, etc. They have to be able to operate over a range of speeds to be able to control the vehicle speed. This range is kept pretty small these days, again to optimize efficiency and emissions, hence the need for multi-speed transmissions. There is not much different in the hardware between a constant speed and variable speed diesel, the biggest is the design/programing of the mechanical/electronic governor. Small details such as the turbo match can be tweaked for constant speed.

One of the principal advantages of the diesel cycle is the lack of intake throttling, this reduces pumping losses significantly. Load control is accomplished by controlling fuel quantity, hence air fuel ratios vary widely, something not possible in spark ignition engines.

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RE: Difference between constant speed and variable speed engines

dgallup has it right in his discussion of const. vs. var speed., although there are some constant speed mechanical drive applications out there too.

RE: Difference between constant speed and variable speed engines

Quote (ivymike)

dgallup has it right in his discussion of const. vs. var speed., although there are some constant speed mechanical drive applications out there too.
This is true, in that these applications tend to run at a constant speed for an extended period of time. However, the speed setting may be changed from time to time in order to adjust to changing operating conditions or external demand.
There also exist generator applications that are switchable between 50 and 60 Hz, that require the engine to operate efficiently at two different speeds separated by this ratio. I'm thinking of rental generator packages, that can be deployed in areas with either of the two standard grid frequencies.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

RE: Difference between constant speed and variable speed engines

(OP)
thank you everyone for the comments , those really helped.

One more thing I want to ask here that why diesel engines don't need a throttle ?(sorry but I am a bit naive)

RE: Difference between constant speed and variable speed engines

2
A better question is "why do spark ignition engines need a throttle?"

Answer. To reduce engine output at a given operating speed, you obviously need to reduce the fuel (energy) input. A SI engine can not ignite and burn the very lean mixture that would result, so the airflow needs to be reduced in line with the fuel reduction.

In a Diesel engine, a combustible mixture will always exist somewhere between the injector (100% fuel) and the combustion chamber furthest from the injector (100% air), so it can be operated with maximum airflow even at reduced output.

Engineering is the art of creating things you need, from things you can get.

RE: Difference between constant speed and variable speed engines

I've never heard the question turned around like that, ("why do spark ignition engines need a throttle?"), but it really is a much better question.

- Steve

RE: Difference between constant speed and variable speed engines

(OP)
Thank you gruntguru , that was really helpful and precise.
Thank you again

RE: Difference between constant speed and variable speed engines

Both SI and CI recip engines use some form of throttle. SI gasoline engines throttle the amount of intake air needed to produce a correct A/F mixture which will create the amount of power needed to match engine load demand. CI diesel engines throttle the amount of fuel needed to produce the amount of power required to match engine load and speed.

RE: Difference between constant speed and variable speed engines

Trying to think of engines that aren't untimately controlled by throttling (fluid flow through variable area orifice).

I suppose fuel quantitiy in a diesel pump-line-nozzle system is often regulated via some kind of upstream control valve. And in common rail systems with actuated needle lift, flow is quantity is regulated at the nozzle, so it is sort of throttled.

But is fuel "throttled" in a diesel engine that's regulated by the height of a helical spill port? Height controlled directly by rack position.

- Steve

RE: Difference between constant speed and variable speed engines

... or regulated by the duration of an injector pulse, as with a modern common-rail?

RE: Difference between constant speed and variable speed engines

Yeah, but even with an injector pulse, the quantity is still dictated by fluid mechanics: differential pressure across an orifice. It's just semantics. But no more so than stating that all engines use some form of throttle.

- Steve

RE: Difference between constant speed and variable speed engines

The runaway diesels in the linked videos ran away because the intended "throttling" of the fuel was hoisted by the petard of combustible stuff entering from elsewhere. A very leaky turbocharger lube circuit , for instance.

The solution is to apply an air throttle over the intake of the runaway Deezul..
In one of the videos the young tech gets scared and runs away when the engine intake tugs violently at the wad of cloth he tries as a plug.
The old timer then approaches with a hunk of wood that effectively resists being sucked in.

Semi modern VW TDIs add a valve to the intake tract.
http://www.myturbodiesel.com/images/runawayengine....\
http://www.myturbodiesel.com/wiki/diesel-tdi-engin...
the main page say the valve's main purpose may be to reduce shudder in some operating conditions, but it looks like it may also be snapped shut as part of the shutdown procedure.

This link (and others) suggests all 71 series Detroit Diesels (Supercharged 2 strokes) had an "emergency stop" flap in the intake, operated from the dash, but that it's use might cause such high vacuum that supercharger seals were damaged.
http://www.thedieselstop.com/forums/f33/6-71-troub...

RE: Difference between constant speed and variable speed engines

Talking about runaway diesels, inert gas (CO2) injection is also starting to become a more popular method than slamming the intake air shut-off closed on heavy duty hazardous environment diesels, as it avoids some of the issues with sudden vacuums damaging seals, etc. There are issues with being able to restart the engine after an E-stop (the bottle needs replacing/recharging) but some manufacturers would rather that than damage an engine.

RE: Difference between constant speed and variable speed engines

Whoops! By injection, I mean injection into the intake manifold to stall the engine.

RE: Difference between constant speed and variable speed engines

There are a lot of sound reasons to maintain constant speed (RPM). My favorite used to be the C-130 Hercules aircraft - considered by many to be the world's most successful cargo aircraft. During operation, the four turbojets run constant speed no matter what activity if performed. To adjust power output, the pitch of the blades is moderated, and fuel consumption rate is adjusted to present specific available power to the blades.
Four very strong engines on an airframe are very easy to coordinate. Vibration quotients throughout the craft are more stable, and the engine life is elongated through constant speed. Ancillary systems are tuned to that RPM, and the system is more efficient.
For as much as any power system may be "notched" into a specific portion of its power curve - never leaving that notch, constant RPM can serve as an endurance factor. By moderating fuel input & mixture (including turbo boost), cam timing, and torque transfer, very efficient operation is possible. If the notch is selected in the most favorable portion of the power curve for application, you can expect the maximum-available power at peak load, and moderate economy during light load. Idling is a loss factor, unless the controls 'step' from true idle to power.

RE: Difference between constant speed and variable speed engines

That's an excellent point about turboshaft engines (or turbine engines in general). A recip diesel engine can readily operate at a wide range of speeds, but they are often optimized to operate at a specific speed when used to drive a synchronous generator or pump. Turboshaft or turbofan engines on the other hand can only operate over a fairly narrow range of speed, usually somewhere around 80%-100% of their rated speed. If you operate the engine much outside of this speed range, its combustion can become unstable. With turboshaft engine helicopters, the engine operating speed range becomes even more restricted due to dynamic issues with the rotor system.

RE: Difference between constant speed and variable speed engines

This is true, in that these applications tend to run at a constant speed for an extended period of time. However, the speed setting may be changed from time to time in order to adjust to changing operating conditions or external demand.

That's not what I meant - there are actual constant speed diesels for mechanical drive applications which have a non-adjustable fixed operating speed (1800 rpm in one example). These engines can only operate at low idle (for warmup and shutdown) and the rated speed. The engine software and legal requirements (emissions certifications) prevent operation at any other speed. Lest you go away thinking that emissions certifications were the only thing that drove creation of these engines, the application existed before the emissions-certified engine was developed (or it wouldn't have been).

A constant-speed engine can be tailored to give better transient load response at the desired operating speed, which helps in gensets and also in some mechanical drive applications.



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