cylinder scale
cylinder scale
(OP)
What happens to efficiency and output of a mono cylinder engine if it is made similar, but with half the displacement?
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RE: cylinder scale
What exactly do you plan to change, bore, stroke, both in proportion, ports valves?
Regards
Pat
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RE: cylinder scale
RE: cylinder scale
Same stress - same piston speed, same heat loss (specific)
RE: cylinder scale
All dimensions in this hypothetical engine are reduced to 0.7937... of their previous values. Call it 80% for round numbers.
If you want to maintain comparable stress levels that implies comparable piston acceleration and piston speed. This means it will be permissible to rev about 25% higher (1 / 0.7937...).
If cylinder filling is kept in the same range at this nominally higher speed - which ought to be at least approximately the case, if all cylinder port and intake runner dimensions are reduced by the same factor, which will maintain all flow velocities the same and since the speed of sound is the same, the pressure waves are travelling a shorter distance through the intake runners - then the engine will make approximately 62% of the original power output. (50% of the original torque but at 25% higher revs.)
Efficiency should stay in the same range. There are certain effects that don't fully scale in direct proportion. which will tend towards the smaller engine having ever so slightly poorer thermal efficiency under comparable (dimensionally scaled) load conditions.
Natural question arises - "But if this is the case - why does a car with a smaller engine normally get better fuel economy?" - and the answer is that normal automotive engines are considerably oversized for their actual applications. The smaller engine will be less oversized, and if it's under the same actual load (only a fraction of the actual maximum power output) it will generally be running at a more favorable point on the BSFC map.
RE: cylinder scale
Further to your very well thought out answer, the smaller engine is also lighter and has less torque so can use a lighter transmission and even maybe lighter sub-frame in the engine/transmission mounting areas.
It might even be more compact and allow for a slightly lower bonnet (hood) line and better aerodynamics.
Regards
Pat
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RE: cylinder scale
- Steve
RE: cylinder scale
The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.
RE: cylinder scale
RE: cylinder scale
h.p. = (D2 x n)/2.5
where D = the diameter of the cylinder in inches [1" = 25.4mm], and n = the number of cylinders
The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor
RE: cylinder scale
where D = the diameter of the cylinder in inches [1" = 25.4mm], and n = the number of cylinders
and there was a tax based on this calculated value http://en.wikipedia.org/wiki/Tax_horsepower
and my point is that the tax was proportional to cylinder area, not displacement, which encouraged long stroke, small bore engines to minimize the ratio of tax to actual horsepower (or at least torque!).
RE: cylinder scale
RE: cylinder scale
Cylinder volume, count, bore, & stroke are all compromises intended to achieve the best commercial outcome for the application. It's not surprising that engines competing against each other tend to be very similar in these and other parameters. Even though their designs are sometimes perverted due to arbitrary rules/restrictions and other external factors, this is also true of race engines, e.g. F1.
RE: cylinder scale
RE: cylinder scale
Think of the power output to friction ratio as the displacement increases (frictional losses)
the surface area to volume ratio (heat loss)
also with larger engines, you generally operate at a lower RPMs, which generally means more time for the atomized fuel to evaporate and fully mix, resulting in a more homogenous fuel/air mixture. There is also a much lower volume ratio of "low turbulence areas" in nooks and crannies of the combustion chamber in larger engines than smaller, which further increases combustion efficiency.
Some of the largest diesel engines in the world are also some of the most thermally efficient (somewhere around 50%) as where smaller, lower compression ratio gasoline engines run in the 20% range (I think, I'm going off of what I remember). So in general, smaller engines of similar design will be slightly less efficient due to a combination of different things all working together.
RE: cylinder scale
An engine that is oversized for its application - and that includes almost all current auto engines - will end up running in a less-favourable, light-load part of the BSFC map, and this effect by far overwhelms the effects described above.
RE: cylinder scale
RE: cylinder scale