Possibly stupid question, what's the point of BHP per liter?

[Drakelee117]

BHP per liter is sometimes used as an important standard for an engine. But what's the point for this number since it doesn't take the number of cylinders in account? I mean, how is a 2 liter 4 cylinder engine like the one the twins have comparable to those 5 liter V8s just because they produce similar BHP per liter number? And this next question might sound stupid, but if cylinder number does not matter, why don't car makers make 8 liter 4 cylinders that kind of crazy stuff?

[tyrantcf]

It is specific output per unit of displacement.
Useful for comparison purposes with other data and against other engines to help determine overall efficiency.

An engine with a higher specific output per unit of displacement is more efficient in terms of making power.

And 8 liter 4 cylinder? Because it would be fuckin huge? one head would displace what the whole fa20 displaces. Would be cool though!

[Beil]

well, with bigger engines, the number of cylinders tend to grow for a variety of reasons.

This is just my opinion as an engineer, but if you had 4 REALLY BIG cylinders instead of 8-10 smaller ones, you'd run into problems with the fuel/air mixture not completely burning up in time. To counteract this, you'd have to slow the RPM's down drastically to allow the fuel/air to burn completely. Or you could always have hotter spark plugs, but that will also lead to higher overall heat in the engine(which opens another different can of worms.)
Also, with the larger and heavier pistons, the stresses on the connecting rods and crankshaft are more concentrated to a specific area, so you'd have to beef those up as well, leading to an unnecessarily heavy setup.

I could be completely wrong on this, but I think there is a way of optimizing the number of cylinders with displacement. Fewer cylinders does make for fewer moving parts and more simple design, but the forces are concentrated on fewer moving parts. More moving parts lowers stress concentrations, but also makes for a much more complicated design.

with that being said, would I love to see an 8 liter 4-cylinder? hmmmm....pistons bigger than my head? sounds like fun!!!

[extrashaky]

Originally Posted by Beil

with that being said, would I love to see an 8 liter 4-cylinder? hmmmm....pistons bigger than my head? sounds like fun!!!

How 'bout a 28 liter 4 cylinder?

[ame="https://www.youtube.com/watch?v=PlPbhGeMu4U"]Fiat S76. 1911. 28.4 litres. 4 cylinders. Start-up 28th Nov 2014. - YouTube[/ame]

[cdrazic93]

Originally Posted by Beil

with that being said, would I love to see an 8 liter 4-cylinder? hmmmm....pistons bigger than my head? sounds like fun!!!

do NOT throw those rods....lol

[nzer]

Originally Posted by extrashaky

How 'bout a 28 liter 4 cylinder?

Holy moly. Is that an engine or an out of tune missile!

Got a link to it being driven?

[retoocs]

It's about efficiency. American engine builders try to shoot for 1 hp per cubic inch. The rest of the world tries to get 100 hp / liter. Do the math, which is a more efficient engine?

[mav1178]

Because engineering. Over 100 years of engineering.

+ accounting.

-alex

[swarb]

Also racing. Rules limit engine sizes so if you make more power, you are potentially faster. And in some countries, there is a tax for bigger engines.

[Captain Snooze]

My impression is that power/capacity is a relic from years gone by. Back when peak power was all that mattered because the spread was so narrow the maximum dictated performance.
This is just conjecture on my part.

[Drakelee117]

Originally Posted by Beil

well, with bigger engines, the number of cylinders tend to grow for a variety of reasons.

This is just my opinion as an engineer, but if you had 4 REALLY BIG cylinders instead of 8-10 smaller ones, you'd run into problems with the fuel/air mixture not completely burning up in time. To counteract this, you'd have to slow the RPM's down drastically to allow the fuel/air to burn completely. Or you could always have hotter spark plugs, but that will also lead to higher overall heat in the engine(which opens another different can of worms.)
Also, with the larger and heavier pistons, the stresses on the connecting rods and crankshaft are more concentrated to a specific area, so you'd have to beef those up as well, leading to an unnecessarily heavy setup.

I could be completely wrong on this, but I think there is a way of optimizing the number of cylinders with displacement. Fewer cylinders does make for fewer moving parts and more simple design, but the forces are concentrated on fewer moving parts. More moving parts lowers stress concentrations, but also makes for a much more complicated design.

with that being said, would I love to see an 8 liter 4-cylinder? hmmmm....pistons bigger than my head? sounds like fun!!!

Which has a bigger effect on power output? And how? Number of cylinders or displacement

[navanodd]

Originally Posted by Drakelee117

Which has a bigger effect on power output? And how? Number of cylinders or displacement

Displacement.


The amount of power you can produce is primarily determined by how much fuel you can burn.


It takes 15 parts air to burn 1 part fuel (by mass) and the biggest struggle with any engine is getting enough air in so that you can burn the amount of fuel you want to burn.


The bigger the engine displacement, the more air in the engine to burn the fuel, so the more energy you get per each cycle.


The only other fundamental way to get more air into an engine is to compress it, this is where turbo or superchargers come in.


Now, what I've mentioned above is true for one engine cycle, or assuming that each engine type is rotating at the same speed. If you double an engine's speed you (ideally but not quite true in reality) double to rate at which it intakes air and produce more power that way.


There are of course a bunch of different efficiencies that affect the overall power that the engine will produce for a given amount of fuel (# of cylinders would impact these efficiencies), but basically there are 3 ways to make an engine have more power:
1. Make it bigger
2. Turbo/supercharge
3. Make it faster

[Amaya]

Power/displacement is used to determine how efficient an engine is.
Number of cylinders, bore, stroke, compression, and camshaft profile will be determined by and determine what the engine is used for. Whether it's low revs/high torque or high speed/low torque. There's obviously more to it but in general certain configurations are better for certain applications.

[FRSBRZGT86FAN]

Originally Posted by retoocs

It's about efficiency. American engine builders try to shoot for 1 hp per cubic inch. The rest of the world tries to get 100 hp / liter. Do the math, which is a more efficient engine?

That's not true anymore in the modern American car market....