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General question about spark-ignition engines.
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Guest
Anonymous Poster
andre.zoe@xtra.co.nz
May 26, 2007, 6:59 PM
Post #1 of 4
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General question about spark-ignition engines.
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Could someone with abit of aptitude tell me why, torque, output and car efficiency decreases in spark ignition engines at around 2500rpm? Thanks
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
May 26, 2007, 7:22 PM
Post #2 of 4
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Re: General question about spark-ignition engines.
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I don't believe that's true. It is a challenge to engineer an engine to fully atomize the fuel at the proper ratio and to spark it at the precise time to maximize output torque/HP while maintaining efficiency. Higher RPMs is just a way to use the same displacement more often which effectively makes it behave like and have power like a higher displacement engine would at lower RPMs ,
T
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Guest
Anonymous Poster
May 26, 2007, 7:33 PM
Post #3 of 4
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Re: General question about spark-ignition engines.
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Ok, lets say hypothetically that if we had an engine that was sparking and running with fully optmised fuel/air ratios, at the correct timing have better or worse output,torque, and efficiency compared to that above or below 2500rpm? Thanks
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
May 27, 2007, 5:33 AM
Post #4 of 4
(1333 views)
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Re: General question about spark-ignition engines.
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I'm not sure 2,500 RPM is an exact barrier line to cross. The internal combustion engine when first introduced did not use high RPMs no doubt because balancing parts was difficult. Ability to balance, and lubricate ever faster moving parts which would have much higher demands than a slower turning engine. Metalurgy has raised the bar significantly over the decades. The line of efficiency vs weight has no doubt gone up over the years.
In essence, a huge heavy engine could produce the power with few RPMs but at some point the mere weight of the thing becomes the problem.
Take huge ships as an example. Super huge displacement engines that spin slowly which is more dependable and durable and in a ship the weight is less important. In aviation the weight of the engine is all important conversely.
Sparked engines lose out to diesel types which utilizes "heat of compression" for ignition but you won't find diesel aircraft.
Take the two stroke engine: For the same weight you get twice the power as it fires every time it's on TDC vs every other in a four stroke engine. This is real handy when weight is important like chainsaws, mopeds, and an assortment of equipment that must be carried around but this is at the expense of efficiency. Then at some point the weight of the fuel needed becomes a problem.
Forces are exponential. Doubling speed creates three times the resistance. Try putting your hand out the window of a car at 20 mph, then again at 40 mph. Both tolerable but keep doubling and at some point the force would tip over the car!
The perfect speed for power and efficiency is highly dependant on the application. The perfect line where it crosses will change with technology.
OK. That's my dissertation for the day! We should probably move this thread out of "general discussions" but I'm not sure where to yet,
T
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