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.
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
RE: Difference between constant speed and variable speed engines
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
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
RE: Difference between constant speed and variable speed engines
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
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
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
- Steve
RE: Difference between constant speed and variable speed engines
Thank you again
RE: Difference between constant speed and variable speed engines
http://powernationtv.com/post/2015-gmc-duramax-die...
https://www.youtube.com/watch?v=rLzf3ItkbOE
RE: Difference between constant speed and variable speed engines
RE: Difference between constant speed and variable speed engines
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
RE: Difference between constant speed and variable speed engines
- Steve
RE: Difference between constant speed and variable speed engines
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
RE: Difference between constant speed and variable speed engines
RE: Difference between constant speed and variable speed engines
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
RE: Difference between constant speed and variable speed engines
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.