What is more efficient

[thexav]

Is a small engine more EFFICIENT than big engine? Is a V6 essentially just 2 3 cylinder engines bolted together?

What is it mainly that determines the efficiency of an engine?

 

[Munday]

Is a small engine more EFFICIENT than big engine? Is a V6 essentially just 2 3 cylinder engines bolted together?

What is it mainly that determines the efficiency of an engine?

Assuming they are identical apart from engine size, the smaller one is more efficient due to less internal friction...add a turbo to the equation and your efficiency goes up again.

What you want to be looking at is Volumetric Efficiency (VE) and Brake Mean Effective Pressure (BMEP)

 

[HDi fun]

Thermodynamic efficiency is the overall decider

 

[pgarner]

whats the BEMB ? have heard of the other 2

 

[HDi fun]

BMEP is brake mean effective [cylinder] pressure. It's the thing that gives diesels their fuel efficiency. The cylinders are always charged fully (no throttle) and the compression ratio is high so the overall average cylinder pressure is high. What I don't fully grasp is HOW this makes them more efficient. You'd think that compressing air and then uncompressing it again would be wasteful.

 

[thexav]

So which would be more efficient out of a 2.0V6 and a 2.0V12? Or doesn't it work like that?

 

[HDi fun]

So which would be more efficient out of a 2.0V6 and a 2.0V12? Or doesn't it work like that?

I suspect that the 2.0V6 would win otherwise we'd have a plethora of small capacity V12 engines from which to choose.

THere's also the fact that a 12 cylinder 2 litre engine would require twice as many components, all of which would be half the size (either by area or linear measurement) and as such the manufacturing costs would go through the roof. As would the CO2 emissions of the factory making 2.0 V12 engines because of the number of components involved.

A 2.0 V12 would be very heavy in relation to it's specific power output.

It's better to compare a straight six with a V12 as a straight six is inherently perfectly balanced - no need for additional balance shafts etc.

Clearly a V12 is two of the same so smoothness is assured.

 

[thexav]

Cheers for that its making more sense now.

 

[prevtec]

more cylinders,= more petrol = less efficent

 

[HDi fun]

more cylinders,= more petrol = less efficent

THat's generally the case, but you have to attribute some of this to the fact that more cylinders often means larger dwept volume.

I doubt that the different between a 2.4 litre inline 4 and a 2.4 inline 6 or V6 would be that great in terms of fuel use.

Less cylinders usually means better low down torque, but I'm not entirely sure why this is.

Driving style is still the biggest factor.

A thrashed 1.6 Astra will eat more fuel than a sensibly driven 3.5 V8 BMW.

 

[prevtec]

THat's generally the case, but you have to attribute some of this to the fact that more cylinders often means larger dwept volume.

I doubt that the different between a 2.4 litre inline 4 and a 2.4 inline 6 or V6 would be that great in terms of fuel use.

Less cylinders usually means better low down torque, but I'm not entirely sure why this is.

Driving style is still the biggest factor.

A thrashed 1.6 Astra will eat more fuel than a sensibly driven 3.5 V8 BMW.

as proven on top gear with the m3 and the prius,

and as i said, the way the car is looked after is a big factor

 

[HDi fun]

Even if we're not talking about engine wear by thrashing (and most engines will take severe punishment these days) they all have a 'frame' within which they're designed to be driven.

So you need to buy a car with an engine that suits your driving style to get the best out of your fuel.