Perhaps I am misunderstanding, but it seems the OPs question was not about GenSet characteristics but the effect of RMP of BSFC. Assuming I have the OPs intent correct, I offer this...
Though they are related it's easy to get confused when talking about BSFC versus efficiency. BSFC in the US is commonly stated in units of lb/hr/HP (where lb is pounds of fuel) and HP=RPM*Torque/5252 (see discussion at the bottom of
I highly recommend this site to those learning about the details of engines. It's very informative and doesn't gloss over the details much). In contrast, efficiency is a simple measure of work out (Joules) versus energy in (Joules) and is much easier to follow IMHO.
Positive factors in the efficiency equation are compression ratio, burn rate and/or piston speed (more later), and expansion ratio. Negative factors include heat loss, pumping losses, mechanical losses, and friction. Compression ratio plays into the equation because work out is maximized by expansion and starting with a very small volume at the start of combustion naturally leads to a larger expansion ratio. Burn rate plays into the equation because we want the maximum period of expansion and, since the piston is moving down at combustion, some of the fuel burn occurs at volumes greater than the minimum with negative impact on expansion. You can see how piston speed plays into this equation; a slow burn rate and a fast piston speed means more combustion is occurring at larger volumes, so the expansion ratio suffers. Expansion ratio is obviously key to extracting work from combustion, but it is limited by friction; if expansion chamber pressure is less than that required to overcome friction, the engine is *expending* work to move the piston downward.
Note there is a paradox in dealing with burn rate and piston speed; A slow moving piston ensures combustion occurs when the chamber is closer to minimum volume, and that's good. On the other hand, a slow piston must contain pressure over a longer period and it loses some around the rings (blow-by). Furthermore, a slow piston implies the heat of combustion is exposed to cylinder walls and pistons longer, so more heat is lost. There is a way to work around this paradox by using Homogeneous Charge Compression Ignition (HCCI), but that's outside the scope of this discussion. See my comments under my "Create The Future" contest entry at
for more information. Please register and vote for me while there !
Heat loss affects efficiency in the obvious way; engines are extracting work from heat, so losing heat is bad. Pumping losses are produced by the work required to draw intake charge into the cylinder and push exhaust back out. Mechanical losses result from the need to drive required equipment such as oil pumps and valve trains. Friction is an obvious detractor to efficiency.
Now lets look at spark ignition engines and diesel engines.
Spark ignition engines are typically controlled by limiting air into the cylinder (aka "throttling") and that creates large pumping losses. They then compress the charge as much as possible but are limited by knocking (aka pre-ignition) and thus can't attain very high compression ratios. Once the piston passes TDC, the spark fires and combustion begins (note most piston engines use high octane and/or spark advance to maximize compression before bulk combustion occurs, but that's a detail). Since the intake charge is well mixed, burn rate is determined almost exclusively by flame propagation speed which is pretty fast. In addition, the piston moves slowly near top dead center, so combustion occurs close to the minimum volume and expansion is maximized. Examining a plot of work out in a spark engine reveals an issue with crankshafts, however; they take a while before they really start producing torque, so they spend too much time containing combustion gasses before extracting work and suffer from blow-by and heat loss as a result.
Diesel engines are regulated by controlling the amount of fuel in the chamber and are not throttled, a plus for efficiency. They compress intake air before fuel is injected, and this allows for very high compression ratio. Once they pass TDC, they begin injecting fuel, and it ignites upon contact with the extremely hot air created by compression. This process is inefficient for two reasons. First, combustion occurs along the leading edge of the fuel spray where the charge is very rich and is neither complete nor clean. Second, the spray takes longer than flame propagation, so more combustion occurs at other than the minimum volume.
Now let's consider the impact of RPM on all this.
Engines running at high RPM are designed as air pumps such that gasses in the intake manifold have significant momentum, and this reduces pumping losses. On the downside, high RPM yields greater inertial loads on structures because the pistons are moving so fast. For this reason, high RPM engines typically employ less stroke in order to reduce inertial loads and compensate by using wider bores, but this increases combustion chamber surface area and heat loss. These high RPM engines still have good compression, but expansion is less because stroke is short and the exhaust valves have to open sooner. The reduced expansion ratio results in a lot of wasted heat/pressure in the exhaust. Note nearly all engines, whether spark or diesel, typically try to recover some of the lost energy resulting from less than complete expansion by using turbochargers (which reduce pumping loss and, if desired, apparent displacement).
The bottom line of all this is that diesels tend to be more efficient because the losses resulting from extended combustion are more than overcome by gains in compression and expansion ratio. The long strokes they use to gain increased compression/expansion, however, drives lower piston speeds to reduce inertial loads, and this increases heat loss. Diesels also tend to be quite heavy because their high compression ration results in higher combustion temperatures which drives a need for beefier blocks, heads, pistons, etc.
Which is better ? It depends on the application.
I hope this helps. Sorry if I rambled on too much !
Rod
