Quote:
Originally Posted by Ken Fry
So the larger engine would consume 290/230 or 1.26 times as much fuel.
At lower cruise speeds, the difference would be much greater.
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For example: Suppose the car is in town, cruising at 30 mph. The HP used will depend on a lot of factors (parasitic loads, AC on-off, relative effects of rolling resistance vs aero, etc) but an OK guess would be that 4 hp is required in the same car that required 11 hp at 60.
1800 rpm seems like a reasonable rpm for driving around town. 4 hp would require 11.6 lb-ft (15.6 Nm) at 1800 rpm: 500 gr/kWh with the 2000cc engine.
Using the same rpm, with the 500cc engine, the required torque would show up at the 62.4 Nm line (15.6 x 4). At 1800 rpm, we are at about 305 g/kWh.
(This torque scaling is avoided in charts that show BMEP instead of torque, incidentally.)
At this reduced load, the smaller engine is 500/350, or 1.63 times as efficient.
A holistic glance at the chart shows that for both engines, it would be better to drive at 900 rpm, where we'd need double the torque. This improves the efficiency of the bigger engine to maybe 410 gr/kWh. The smaller engine would be at about 260 g/kWh. The comparative ratio is then 410/260 or 1.58 times as efficient for the small engine.
Jack McCornack's car shows the advantage of the combined effects of a small (32hp) engine and a diesel in a car that is of similar mass to yours. He is using both a small (32 HP) engine and a diesel in a small, light car. In his streamlined version, he is getting roughly 80 mpg at around 60, which is pretty impressive.
Because your engine is much larger, and of less efficient technology, you probably can't expect anything very close to Jack's fuel efficiency. However, your solution will cost far less, and will be far cleaner, in terms of NOx, particulates... the whole nine yards. Also, a gallon of diesel is over 8 lbs -- so a gallon of diesel represents more oil. So even if you are down around 65 or 70 mpg (at the same cruise condition that produces 80 mpg in Jack's car) The actual resource consumption (if that matters to you) might not be so large a difference. (As you probably know, this stuff can get quite complicated when you consider the energy invested in the different refining processes, etc etc.)
I understand that you, Doug, are probably familiar with this stuff, but supply this level of detail because others might be just learning about this (contemplating, for example, what size engine to use in the next Vetter challenge) etc. I am a natural pedant, and having taught this stuff, the teacher role pops up. (As a measure of how old I am, when I taught this, 25% was considered pretty good efficiency. The Saturn engine, if the 230 peak is to be believed, is about 36% efficient, using 33.7 kWh per gallon of gas and 6.2 lb.gallon (and the mixed units !! that are required of people in the US these days.)
Regards,
Ken