'Right' cylinder volume

[diluculophile]

Is there an optimal cylinder size with regards to power/torque output and fuel usage?
Is it affected by the presence or absence of turbochargers or superchargers?

Slightly frivolous thread, I know, but for some reason 500cc's seems appropriate:

V-Twin sportsbikes should be 1000cc right?

2 litre 4 pots like that in a RenaultSport Clio RS200 (not the new one) hit the spot

Straight and V6's of about 3 litre displacement? I'm thinking BMW 330i/335i

4.0 V8? - BMW M3 E92, recent Mercs with a twin turbo?

6.0 V12 - Astons.


I'm willing to consider 'as big as possible' as a reasonable argument

Discuss...

[diluculophile]

Jacobyte said:

I read many years ago that 400cc is the optimal volume for most efficient combustion in a 4-stroke engine. I can't recall the source though.

I'm wondering whether that has changed with recent technology - turbo's etc. and the habit of manufacturers downsizing engines.

[diluculophile]

Willy Nilly said:

What application are you talking about OP? 500cc/cylinder is a big lawn mower, but would be st it a lorry.

I was thinking road going (and possibly track?) cars and motorcycles as per the list in the first post.

What size cylinders would be appropriate in a lorry and why?

[diluculophile]

Alias218 said:

Cylinder volume is more than just swept capacity; the combustion chamber adds quite a few cc's to a cylinder's displacement and plays if anything a larger role in getting power/torque/economy than bore/stroke ratio.

For instance, the hemispherical head made famous by Dodge's 426 has superb thermal characteristics owing to it's ability to maximise volume, but minimising surface area. This allows for good heat retention, higher cylinder pressures and hence more power and torque. You can also fit in some large ports, making breathing characteristics equally good. However, with this shape you lack ample squish and swirl zones. This makes for poor fuel/air mixing and thus poor fuel economy.

On the other hand, take a wedge shaped combustion chamber and the thin end of the wedge makes for a brilliant squish point, making combustion easier as temperatures rise through pressure, plus as the squished fuel moves into the fat end of the wedge it swirls and mixes well. The outcome is a good burn and good fuel economy. However, the surface area is quite large for the volume, resulting in poorer thermal retention properties and, you guessed it, lower power and torque outputs.

To say 500cc or whatever is the optimal size is arbitrary analysis. One 500cc cylinder can have vastly different characteristics to another based on combustion chamber design, breathing properties (including exhaust scavenging), injection type and spark positioning relative to the combustion chamber (among other things).

The variations in cylinder displacement amongst manufacturers is testament to this.

Edited by Alias218 on Thursday 27th August 23:04

Makes a lot of sense.
In short, as with almost everything, it's going to result in a compromise.

It seems like your optimal cylinder size and shape will be dictated by your intended application.

[diluculophile]

TooMany2cvs said:

diluculophile said:

I was thinking road going (and possibly track?) cars and motorcycles as per the list in the first post.

I'd suggest that 500cc/cylinder was WAY above average for road-going motorcycles.

The number of 1000cc+ twins and 500cc singles is MUCH smaller than the number of 50cc and 125cc singles, and even the number of bigger bikes with 600cc twins/triples/fours to 1000cc triples/fours.

Very true, the sportsbike market does seem to be dominated by 600 and 1000cc multi's.
I just happen to like big V-twins - Ducati 999 etc, Aprilia RSV (60 degree v-twin?), Honda SP1 and SP2 etc. etc.

I can't really argue that a big twin is better - the multi's tend to have better outputs and so on.

In a smaller and lighter application where torque is probably less important and top end is seen as desirable, perhaps smaller cylinders are king?

[diluculophile]

nedge2k said:

diluculophile said:

Is there an optimal cylinder size with regards to power/torque output and fuel usage?

I think in general the answer here is no.

Power, torque and MPG are all dependant on a variety of factors and most people only have the experience of production engines to infer from. Production engines are a massive compromise - emissions, NVH, cost, longevity, taxes, packaging etc. all play their part in holding back an engine from what could be optimum.

Cylinder volume is not quite the right question. You could have a 86x86mm bore/stroke or 95.28x69.24mm or 87.5x82 or 85x88mm - all would be near enough the same CC when you account for the combustion chamber but all would perform differently - even if you assumed all had the same size/type/number of valves, same port lengths/shapes/diameters, same friction losses in the valve train/bottom end, lightest possible rods/pistons for their size, etc. etc.

Also, it also depends on your idea of "right"? It makes mega power and gets decent fuel economy but only lasts five minutes - is that right? Would be for some...

If you really get into how an engine works, the possibilities for performance, the costs, the restrictions, the required longevity you'll realise there is no answer to this question. Scientifically there may be but practically, there won't be.

Edited by nedge2k on Friday 28th August 11:39

Down to application again - An F1 or MotoGP engine only has to last for a race or two, but if your suped up Impreza went bang every 500 miles, you'd be pretty pissed off.