goodvibes

cummins power and darkfire
sorry its drifted away from , who gets, etc, the only reason i mentioned a diesel was to show in a simple way why a small petrol engine at full throttle works more efficientlly than a big engine at small throttle, sorry to have bored you with the 'how and why' happy motoring, mike.
baron brian, eddie c and any one else who stated that ' the compression ratio'
cant change are , of course correct, stupid mistake, what i meant to say is that
' the petrol engine is a variable compression engine' which it most definitely is.
the compression,[ psi] is high only at wide open throttle, because there is no restriction to the cylinder filling with air at [almost] atmospheric pressure.
at lower throttle openings , the compression drops sharply , untill at tickover
there is virtually no compression at all , but fortunately the spark can still ignite
the mixture.
baron brian, you said ,'a diesel is more efficient due to fuel energy density and
higher burn temperature' not true, these are contributory factors not the main one.
put, 'what is the major factor in diesel engine efficiency ' into 'google' and every reply ive seen all state , ' that its the very high compression ratio' which is responsible.
you also said,' you can stop with 'diesels dont have a throttle and thats why they are more efficient' what i actually said was' thats ONE of the reasons a diesel will hammer a petrol on mpg, it doesnt have a throttle so it operates at max compression ALL the time, the other reason of course is the comp ratio is about twice as high as a petrol'. both of these reasons are true.
you mentioned my ' first post of misinformation on page 2' apart from my stupid mistake about saying 'compression ratio' instead of using the word compression, id like you to point out anything else in that post which is not true, feel free to prove me wrong , im a lover of the truth.
racing is about power and acceleration and is virtually the opposite of driving for efficiency, it bears little relation to driving on the road.
driving through town at slow speeds the bmw's big engine would have the throttle barely open, sounds good for economy, but wait, if the throttle's barely open, the cylinder compression will be very low, thats low efficiency.
the small engined car following , provided its in the highest gear it s comfortable with, will need a larger throttle opening, sounds bad for economy, but wait, if the
throttle has a larger opening, the cylinder compression will be higher,
that means higher efficiency.
at moderate speeds, provided the small car always uses the highest gear its comfortable with,the throttle opening wil have to be opened a bit more, again it sounds bad for economy but this will further increase the cylinder pressure increasing the efficiency again.
at some higher speed, the smaller engine will reach max torque rpm,[this is usually in the handbook] for max efficiency, this rpm must not be exceeded because the cylinder compression starts to drop after this point, and efficiency
drops, but the power will increase if the throttle is opened more.
up to the point where the small car reaches max torque , mr bmw will always
be operating at a relatively smaller throttle opening, hence lower compression
and lower efficiency.
big petrol engines [and small] at low throttle openings are effectively 'low compression engines' they only turn into' high compression' high efficiency
engines at high throttle openings, but how often would you use full throttle
on a really big engine? .
mike.