Engine Displacement and Cylinders

[TrvlynAlec]

"There is no replacement for displacement" is what many people say, but many times it is not so. Some engines like the lamborgini have 6.0L and 12 cylinders. Others like the Honda F-1 race car is a 1.5L V6. My father has a Chevy 6.5L turbo disel V-8(i think). The honda prelude is a 2.2L 4 cylinder. I dont get why some engines have small displacement any many cylinders. And others have a large displacement and few cylinders. Why do they do this, and what are the advantages of each? Thanks

[Steel]

well there really IS no replacement for displacement , just that with the more cylinders, the engine runs smoother and iirc, a bit more efficiently. If you had a 6.1 litre 4 banger, your car would be jumping all over the road. But it depends on the manufacturer. usually 0-2.5 litres is 4 cylinder, 2.6-4.0 is 6, 4.1-5.7 is 8 (with exceptions) like your dad's v8 diesel. the 6's are usually race engines used by v12's excpet the Viper with has an 8 liter V10, or a hemi 528 which is an 8.6 liter V8!! frickin big cylinders i tell you now! There ARE 3 and 5 cylinder engines too, so dont forget those. Like i said, it depends on the manufacturer and what they want out of the engine.

[ivymike1031]

well heck, there used to be a 66L (not 6.6L) 3cyl sitting about 100ft from my desk, but they gave it to a university to use for research. It was a running prototype for a 22L/cyl V16. Obviously not well suited for a small vehicle application...

I guess the point is just that there are different constraints for different designs. Often a very slow moving engine can have really high displacement per cylinder, and medium to high speed engines have lower displacement per cylinder. More cylinders can mean more cost to build the engine, but having less cylinders can limit power output and/or engine speed... there are really lots of factors that go into the decision.

[enginerd]

I saw a natural gas pump here once. It was the size of a house and was nothing more than a flat 8. One side was powered by what else but natural gas and the other was a gas pump to get the gas out of the ground.

[TrvlynAlec]

"engine runs smoother and iirc" what is iirc? And if ivymike1031 is correct when he says "having less cylinders can limit power output and/or engine speed" then why is everyone crazy over a 8.6L V8 hemi engine? Does that mean a lamborghini engine is better than a 8.6L V8 engine? I mean it does have more cylinders. Yeah the hemi will have tons of torque, but the lambo should have more overall power/speed becuase of the extra cylinders.

[ivymike1031]

perhaps I should have said "for a given displacement..." I'd say that in general, for a given total engine displacement, an engine with large, slow slugs will produce more torque than an engine with a bunch of little, high-speed pistons, and the high-speed engine will have a higher peak power output. Additionally, in general, the large displacement/cyl, slower engine will be undersquare, while the faster, many-cyl engine will be oversquare.

Before anyone starts in with counter-examples - I realize that if you start comparing dissimilar engine configurations, my general statement will seem incorrect (turbo vs na, diesel vs gas, etc)

[fieroturbo]

Well, yet another topic that's "nucking futs!"

To my knowledge, the reason F-1 racers (An Ferrari 360 Modenas) use more pistons, with less stroke and bore (displacement per cylinder), is because the shorter the stroke, the higher RPM's you can get to. And a smaller bore, or thicker cylinder walls, increase reliablity of the block.

Now, trucks (pickups or semi's) perform their duties better with fewer pistons with larger diameters.

I'm not a scientist, I'm a hobbyist. So I can't tell you why, I can just tell you what.

I do know however, the larger the piston surface, the more surface there is to push down on, which I guess results in more torque. The combinations I've seen on my engine, the Pontiac Iron Duke (both truck engine and NASCAR engine), makes me assume that there's a ratio between Bore and Stroke that is equivalent to Horsepower and Torque.

I'll find a way to explain it. I'll look through the dyno charts I have and see if I can explain it somehow.

[Polygon]

Originally posted by TrvlynAlec
"engine runs smoother and iirc" what is iirc? And if ivymike1031 is correct when he says "having less cylinders can limit power output and/or engine speed" then why is everyone crazy over a 8.6L V8 hemi engine? Does that mean a lamborghini engine is better than a 8.6L V8 engine? I mean it does have more cylinders. Yeah the hemi will have tons of torque, but the lambo should have more overall power/speed becuase of the extra cylinders.

Not so. The Plymouth Superbird in the late 60s/early 70s broke 200 mph with a 426 Hemi under the hood. The 528 Hemi would murder a 426 Hemi. So imagine what the 528 Hemi would do under the hood of the Superbird. The 528 Hemi has over 700HP and 700 torque out of the box and they also use the 528 Hemi in top fuel drag cars and pull 7000HP out of them. So that statement does not always apply but sometimes does.

[enginerd]

Also consider that Formula cars as the name implies must follow a formula. Normally the participants are given a choice between a larger, normally aspirated engine, or a small turbo engine. Almost always the engineers choose the small, turbo engine. 'Normal' conditions do not apply. Pump gas is not fed to formula engines. These are not real world engines that last 100,000 miles. Cylinder bore, stroke, engine configuration and countless other varibles are considered in engine design to best fit the demands and expectations of that engine in it's intended application. Early in American auto engineering, cars were equipped with slow-turning, long stroke, small bore engines. These were ideal for pulling tree stumps, treading through muddy bogs (before modern roads) and hauling supplies. In the middle of the twentieth century, engineers were developing better and better gasoline and auto makers decided to capitalize on it. Engines with a large bore and a small stroke were developed. Horspower figures jumped and the muscle-car era was in full bloom. Later in the twentieth century, engines began to have "square" or close to square dimensions ie. square being bore and stroke being close in numbers. This compromise leads to good gas mileage and a good power. Supply and demand really dictate engineering. We don't want gas hogs or slow turning engines today, so engineers give us what we will buy.

[fieroturbo]

Hey, I think somewhere on this "fup ucked" site is a Formula 1 forum. Maybe they know!

[SaabJohan]

Fewer cylinders means higher efficiency but very few cylinders means problems with the forces created by the reciprocating piston.

The power of an engine depends of the amount of fuel that can be burnt, and the amount of fuel depends of the amount of air.

A naturally aspiranted 6 litre 4-stroke engine at 6000 rpm will use 18'000 litres air / minute if we consider the VE to be 100%.
We can compare that to one of the old F1 engines, 1,5 litre, 12000 rpm and around 4 bar boost pressure. That engine will use 45'000 l /m, that is 2,5 times the amount of air used by the 6l engine.

With tha amount air used per time unit we can estimate the engine power if we know the fuels energy content, SAFR and the engines effiency. In theese cases the power can be estimated to 470 hp and 1180 hp if gasoline is used.

[Steel]

Originally posted by TrvlynAlec
"engine runs smoother and iirc" what is iirc? And if ivymike1031 is correct when he says "having less cylinders can limit power output and/or engine speed" then why is everyone crazy over a 8.6L V8 hemi engine? Does that mean a lamborghini engine is better than a 8.6L V8 engine? I mean it does have more cylinders. Yeah the hemi will have tons of torque, but the lambo should have more overall power/speed becuase of the extra cylinders.

iirc=if i remeber correctly. And people are crazy over 528 hemi's cause anyone who wants one HAS to be crazy (like me) But the sheer power of the engine overcomes the lack of 4 more cyliders, and i never said that the 528 is gonna run as smooth as a baby's ass. Far from even. and it wont spin at nearly as fast as the lambo engines, and THAT's why the lambo's would have a speed advantage. There is a point where enough is enough. There ARE v16's (mostly in old planes) and V24's and other crazy numbers, but after V12, the size and weight of the engine works against itself.

[MBTN]

V12 engine is usually the practical limit in cylinder numbers. An engine with a high number of cylinders (let's say V12) has a smoother firing order, which allows the engine to run smoother than let's say an I4. Given the same displacement:

More cylinders: smoother and higher horse power
Less cylinders: More torque, not as smooth.

[TrvlynAlec]

More cylinders: smoother and higher horse power
Less cylinders: More torque, not as smooth.

Ok..so say if we have a 2.0L engine..one has 4 cylinders and the other has 6 cylinders. Will the 6-banger be able to achieve higher horsepower, even though they both have the same displacement? Second, if you have a engine with a bore bigger than the stroke, will it have higher a top speed than a engine with a stroke bigger than the bore? Thanks for the post

[ivymike1031]

Ok..so say if we have a 2.0L engine..one has 4 cylinders and the other has 6 cylinders. Will the 6-banger be able to achieve higher horsepower, even though they both have the same displacement?

If both engines are consistent with general trends, then yes. There are plenty of factors that could make a given pair of engines go against the general trend, of course.

Second, if you have a engine with a bore bigger than the stroke, will it have higher a top speed than a engine with a stroke bigger than the bore?

(I assume you meant for a given displacement) Yes, in general that is the way things go. A longer stroke means that at a given engine rpm, the pistons will be moving faster, and they'll undergo greater accelerations when they reverse direction. This translates to larger loads on the mechanical components that control the pistons' motion (cylinder wall, wrist pin, conrod, crankshaft, etc). Thus it is generally easier to make the smaller-stroke engine rev higher. As always, there are other factors - a major limiting factor on engine top speed is the ability to maintain control of the valvetrain. If your valvetrain can't run at the higher speeds, then your engine can't (because you'll start to run valves into pistons, etc). The valvetrain speed capability is independent of bore, stroke, and the price of tea in china.