Comparitive efficiency of differently-sized car engines

[Cosmic Roy]

Hi All,

Please forgive me if this is a naive question, but my experience with the theory of internal combustion engines amounts to the last 20 minutes I spent reading about them on howstuffworks.com.

I was thinking about the differences between car engines in terms of total displacement, and I was wondering whether, say, the 5-litre engine in a high-performance sports car necessarily burns more fuel than a 1.2-litre engine in a hatchback, when driving at the same speed. Or, taking the figures published by the manufacturer, why does a 1.4i Duratec petrol Ford Focus achieve 42.8 mpg (combined cycle) while the 1.8i version gets 40.3 mpg? Surely, it's not just that the larger cylinders of the 1.8 version weigh more... is it? I would have thought that, if both cars drive at the same speed, they would burn the same volume of fuel. What does the larger-engined car do with the excess energy liberated by burning additional petrol? (Or is just that the petrol is burned less efficiently?)

Answers will be appreciated!

Thanks,

Roy

[Prometheus]

I'm not an expert at all, but my understanding is that an IC engine reaches it's maximum efficiency at close to it's maximum RPM. The slower the engine is running, the less efficient it is. A smaller engine has to run faster more of the time, so it's more efficient. Some hybrids take advantage of this by using a very small gas engine as a generator to power an electric motor.

[Bikewer]

One has to take fuel-efficiency figures with a rather large grain of salt as well. The testing protocols used by D.O.T. are highly suspect, for instance.

There are a lot of other factors at work; gearing, aerodynamic efficiency of the vehicle's body, tires, etc. Often, at highway speeds, a larger engine may be "loafing" at a reasonable cruising speed, able to maintain say, a considerably lower rpm in a given gear.
However, "opening up" the larger engine causes a huge decrease in efficiency.

[Toke]

Prometheus, got it.
An engines efficiency is among other things dependant on compression pressure.
The more the caburator opens the more air it gets and the higher the pressure.
With diesels more fuel will give higher pressure at combustion and the higher output will run a turbocharger to higher airpressure.

One experiment had a 60Hp diesel compared to a 20Hp that was turbocharged to 60Hp.
The smaller engine reached max efficiency at 20Hp and kept it to 60Hp.
The larger had a gradual increase, with rather low efficiency at 20Hp.

There have been some mechanicaly unusual engines with adjustable compression rates, not commercially succesfull through.

[Pantaz]

There are so many variables... a larger displacement engine will typically have greater frictional losses from such things as larger bearings; larger, stronger components means increased mass of moving parts; more engine power may require stronger (and heavier) drivetrain components; the bigger engine likely requires additional cooling, which may invoke aerodynamic losses as more airflow is directed to the radiator.

The list go on, and on.

[DogB]

Agreed - Too many variables.

A few years ago Top Gear did a silly experiment where they ran a Toyota Prius at it's maximium speed around a test track then matched that speed witha BMW M3. The M3 did 19mpg vs the Prius' 13mpg.

[Ziggurat]

I was thinking about the differences between car engines in terms of total displacement, and I was wondering whether, say, the 5-litre engine in a high-performance sports car necessarily burns more fuel than a 1.2-litre engine in a hatchback, when driving at the same speed. Or, taking the figures published by the manufacturer, why does a 1.4i Duratec petrol Ford Focus achieve 42.8 mpg (combined cycle) while the 1.8i version gets 40.3 mpg? Surely, it's not just that the larger cylinders of the 1.8 version weigh more... is it?

It's probably several factors. First off, a larger engine will almost always have more internal friction per cycle, simply because you've got more surface area in contact. If the two cars have the same transmission, this would likely mean the same RPM's for both cars and so more internal friction (and less efficiency) for the larger engine. But of course, that's not the whole story. The two engines will also have different weights, other components of the car might need to be heavier as well (possibly larger starter motor, larger radiator, etc), and so on. And making a car heavier will increase rolling resistance, which introduces more inefficiencies as well.

Of course, this is specifically for cars being driven in a similar manner. If you pulled the engine off the vehicle, put it on a lab table, and tried to run the engine at maximum efficiency (which involves tuning both the RPM [i]and[i] the power output), I'm not sure the scaling of various quantities would give you much difference in efficiency, and might even favor a bigger engine. But we don't drive that way. We drive to achieve certain speeds and certain accelerations, and we care about how far and how fast we go, not about how many joules of energy the engine output doing so, so even the definition of efficiency is different (miles per gallon versus energy output per energy input).

I'm not an expert at all, but my understanding is that an IC engine reaches it's maximum efficiency at close to it's maximum RPM.

No, its maximum efficiency (as opposed to maximum power output) is usually well below the engineering limit. I think for most gasoline engines it's somewhere in the neighborhood of 3,000 RPM's. Engines often run below this peak efficiency, but they can easily exceed it when accelerating.

But this does bring up a related issue, which is that smaller engines are not guaranteed to be more efficient. If you're using an engine which is too small for the task, then in order to get sufficient power you'll need to run it at speeds well above its peak efficiency. So depending on the task (such as hauling a heavy load), it's possible to lose more from running the engine too fast than you gain from having a smaller engine.

[casebro]

Gasoline engines 'pump' lots of air, The smaller one, needing it's throttle wide open, will be a more efficient pump than a larger engine with it's throttle half closed.

And no energy wasted by pushing bigger and heavier parts up and down, round and round, when a smaller part will do.

And, the larger diameter of the piston comes into play too. It takes time for gas to combust. So long that the normal engine ignites the fuel up to 45 degrees before the piston reaches the top of it's stroke. Some of the pressure is actually pushing the piston down will the piston is still rising. Waste of energy there. And a bigger piston, it takes even longer for the flame to travel the whole width. So, the smaller diameter piston means more of the fuel is burnt at a more optimal time. Smaller bore = greater efficiency.

5" diameter seems to be the practical limit, where diminishing returns play.

That's why modern engine design is leaning towards more pistons- V-12 instead of a bigger. V-8.

[Cosmic Roy]

Thanks to you all for your answers; they are very useful and interesting!