Why less cylinders?

[jeff88]

So it seems the general consensus is to use an engine with less cylinders. This makes sense as using less cylinders means using less gas. My question is why not use more cylinders that are smaller. So for example,

2 Engines, each 1.8 liters:
- Engine 1: 4 cylinder engine (.45 liters per cylinder) (ex. 79x91.5 borexstroke)
- Engine 2: 8 cylinder engine (.225 liters per cylinder) (ex. 39.5x45.75 borexstroke)

In the end, you still have the same size engine, just the bore and stroke is exactly 1/2 the size of Engine 1 with half the cylinders, but double the size cylinder. I'm thinking this might be more efficient as more cylinders will fire per revolution of the crankshaft.

Any thoughts?

[H-Man]

More surface area means more heat loss (bigger clynders have less surface area for a given volume)
A V8 is going to have a lot more mechanical complexity too.

[NHB]

Originally Posted by jeff88

2 Engines, each 1.8 liters:
- Engine 1: 4 cylinder engine (.45 liters per cylinder) (ex. 79x91.5 borexstroke)
- Engine 2: 8 cylinder engine (.225 liters per cylinder) (ex. 39.5x45.75 borexstroke)

In the end, you still have the same size engine, just the bore and stroke is exactly 1/2 the size of Engine 1 with half the cylinders, but double the size cylinder. I'm thinking this might be more efficient as more cylinders will fire per revolution of the crankshaft.

Any thoughts?

Engine 1 has 8 times the displacement of engine 2. In order to reduce displacement 50 % you need just to reduce displacement 50 %. Another option is to divide bore with 2^0,5
(79*2^0,5=59)

Fewer cylinders will give you:

• Less surface area for heat loss
• Fewer moving parts means less mechanical friction (pistons, con rods, valves...)
• Smaller engine (one cylinder with 79 mm bore takes much less space than two cylinders with 59mm bore)

On the other hand, fewer cylinders mean:

• More vibration
• Uneven torque output (bigger flywheel)
• Higher piston speed and/or mass (might be a problem at high revs)

[Vekke]

There is also "optimal" piston/cylinder size so its not wise to build small piston V8 engines because you cannot get so good efficiency small v8 as using with correct size pistons.

Also as people already mentioned more parts mean heavier engine and also more expensive engine. OEM try to cut the manufacturings costs anyway they can. Its cheaper to produce big 4 + cylinder engine without turbo than 3 banger with one (two or even three) turbo (s).

However that 3 cylinder is 20+30% lighter and gives better efficiency so fuel consumption is the last choosing point to downsizing from V8 to smaller engines.

Also that 3 banger breaks down faster so its easier to sell new car or new engine after the warranty is run out when that engine fails. that is not fun for the customer but good business for OEMs if everyone is doing it.

[RobertISaar]

a 2L 4 cylinder that's setup similar(target powerband) to a 2L 6 or 8 cylinder engine will be MUCH cheaper to manufacture, lighter, smaller.... all of which OEMs love to utilize.

if you go to some non-mainstream manufacturer, you might see oddballs in either direction(big 4s, tiny 8s), but for a large volume company, oddballs are too expensive.

[niky]

Engine longevity depends more on how robust the construction is rather than the number of cylinders.

There have been some strange engines over the years... Early Ferraris used 1.6 and 2.0 V12s... And there are big industrial diesels with three, two and even single cylinder layouts.

For modern engines, not only do fewer cylinders mean less friction, they also allow you to more easily package multiple valves, dual spark plugs and/or direct injection hardware. Going down to three cylinders makes it more feasible to make small diesels with direct injection. Vibration may cause longevity issues, or they may not. Four bangers aren't well balanced either, without balancer shafts or DMFs, but they tend to last a long time, anyway.

[Big time]

Originally Posted by jeff88

So it seems the general consensus is to use an engine with less cylinders. This makes sense as using less cylinders means using less gas. My question is why not use more cylinders that are smaller. So for example,

2 Engines, each 1.8 liters:
- Engine 1: 4 cylinder engine (.45 liters per cylinder) (ex. 79x91.5 borexstroke)
- Engine 2: 8 cylinder engine (.225 liters per cylinder) (ex. 39.5x45.75 borexstroke)

In the end, you still have the same size engine, just the bore and stroke is exactly 1/2 the size of Engine 1 with half the cylinders, but double the size cylinder. I'm thinking this might be more efficient as more cylinders will fire per revolution of the crankshaft.

Any thoughts?

I think it's because of manufacturing costs.
Usually people interested in mpg also have a limited budget to purchase the car.
It would be interesting to discuss if a shorter stroke would mean less torque?

[JRMichler]

I have an old SAE paper on exactly this: Bishop, I., "Effect of Design Variables on Friction and Economy," SAE Technical Paper 640807, 1964, doi:10.4271/640807.

For a given power at a given RPM, fewer cylinders have less friction and burn less fuel. But more cylinders with less displacement per cylinder can run higher RPM, and thus get more total power.

When the priority is MPG, the solution is the least number of cylinders that meet overall requirements of smoothness, noise, driveability, etc.

[ksa8907]

Originally Posted by Big time

I think it's because of manufacturing costs.
Usually people interested in mpg also have a limited budget to purchase the car.
It would be interesting to discuss if a shorter stroke would mean less torque?

A shorter stroke generally will result in less torque because you lose leverage on the crankshaft. However, increasing the bore will allow for more pressure from combustion on the piston.

My 3.2 v6 is an 81mm stroke by 92mm bore, peak tq of 220@4400rpm and peak hp of 220@6600rpm. Decent low torque, but is pretty motivating above 4k.

[RobertISaar]

(sigh)

will the bore vs stroke "debate" ever end?

torque is largely a function of displacement(regardless of bore/stroke) and compression(not just static compression, dynamic is the key here).

horsepower is calculated from torque at RPM, nothing more.



with VERY few exceptions(all of which are still labratory engines, IIRC) bore/stroke doesn't change(though # of effective cylinders can in certain engines), but you can certainly alter dynamic compression(and some engines even change static compression, but i don't think any are very common).

nearly everything these days uses VVT, which is a good example of that. throttling also plays in as well.



anyways, point is, if you want more torque, you need more dynamic compression or displacement. if you're looking for more power, you need to be able to stuff large amounts of air(and fuel) into the cylinders at higher RPMs, since the horsepower equation "favors" higher engine speeds(since producing the same amount of torque at 2000 RPM compared to 4000 RPM is a doubling in horsepower).

so, in the end, compression and displacement determine torque, the ability to keep large amounts of air flowing at higher RPMs determines HP. the fact that engines that have more bore than stroke GENERALLY do better at higher RPM is due to it being easier to keep airflow up with larger ports that sacrifice low RPM volumetric efficiency(which plays a big part in dynamic compression). there are methods to keep airflow coming at higher RPM without sacrificing low-RPM operation, but they're less "simple". they are becoming more mainstream though(VVT, turbochargers, multiple intake paths, etc).