quote:
Originally posted by Knightrous
From my understanding (I'm happy to be proved wrong here), fuel usage is more related to engine load then engine rpm.
Fuel usage is very much related to engine load AND RPM, however contrary to your thoughts SPECIFIC fuel consumption decreases with increasing engine load. By specific fuel consumption I am referring to the fuel flow rate per kilowatt of engine power produced, ie. it could be expressed by the unit l/s.kw (litres per second per kilowatt).
So why is that? - I'm glad you asked! (any of you olds out there remember The Curiosity Show??)
Firstly, there's the matter of internal friction in the engine. An engine running under no load (free revving) is still burning fuel but producing no usable power - the specific fuel consumption is therefore infinite. All the power the engine is producing is being used to overcome its own friction. Now from this point if you start to increase throttle but maintain RPM by putting load on the engine you start to bring down the specific fuel consumption. As load increases more the proportion of engine power overcoming friction reduces, further reducing specific fuel consumption. Once under full load the engine's friction only consumes a small percentage of the total power produced. Higher RPMs also equate to more friction, and more power required to overcome this before any useful power is available.
Secondly, there's the fact that fuel mix compressed to a higher pressure prior to combustion produces usable pressure in the cylinder for a higher proportion of the expansion stroke. This is why diesel engines are so fuel efficient, and also why high compression petrol engines return better economy than equivalent low compression engines. Cylinder pressures increase with increasing engine load, which due to the more efficient extraction of energy from the burning fuel results in lower specific fuel consumption too.
Now to throw a big spanner in the works, specific fuel consumption under constant load also tends to follow a similar curve to the engine's torque vs. RPM curve. As volumetric efficiency increases so too does the engine's ability to extract the available energy from the burning fuel.
The net effect of these factors is that the engine will have an RPM point where specific fuel consumption is at its best for cruise, and this will be very low - close to the point where the torque curve starts to flatten after its initial rise from zero. It may take 35kw to maintain a steady 100km/h cruise, and unless it has wild cams any engine will use less fuel producing the required 35kw at 2000RPM than it will at 3000RPM, because it is fighting less internal engine friction and burning fuel more efficiently at higher pressure.
I'd also like to add my 2c to the E153 vs. S54 debate in regards to fitting a 3SGTE. You don't HAVE to use an E153, and indeed a V6 will be just as capable of breaking an S54 as a 3SGTE. Breaking these boxes happens as a result of abuse, so which box you choose depends on how you intend to drive. If you want to dump the clutch at 6000RPM you're gonna break things eventually, but an S54 will take the torque of a mildly tuned 3SGTE forever if it's treated with respect.