Does a more powerful engine use less fuel for a specific load than a less powerful engine?

Question

A colleague told me about a truck driving in the mountains. He said if the engine is too small, it would use more fuel to pull the truck up than if the engine were larger, noting the reason is the smaller engine must work harder to achieve the same as the larger engine.

As far as I understand, a unit of fuel, being diesel or petrol, contains a set amount of energy, and therefore I don't understand why a larger engine would use less fuel than the smaller counterpart, as the amount of fuel needed to give the needed energy should be the same, right? It just seems counter intuitive to me that a larger engine could use less fuel.

So, my question is: Is it true that a larger engine could, potentially, be more fuel efficient than its smaller counterpart? And in case it is true, why?

Answer 1

This is true.

Consider the image of a RAV4 hybrid instrument panel:

In it, on the left, you see that close to 50% of the maximum load, you are in the ECO region and between 50-100% of the maximum load, you are in the PWR region (the upper end of the PWR region is accompanied by loud sounds coming from a high-revving engine).

Or consider the basic engine operating line of a Toyota Prius engine (the contours are g/kWh contours):

You can see that max efficiency is at about 2100 RPM and 100 Nm, whereas maximum power is 5500 RPM and 130 Nm. Max efficiency occurs at 2100*100/(5500*130) = 0.29 times the maximum load. Or, approximately 30%.

Now, why is the ECO region in the RAV4 hybrid up to 50% of max load and not up to 30% of max load? This is explained by the electric boost inherent in hybrids (it provides the 20% gap).

If you don't have a hybrid, then ideally, you should be using 30% of the maximum power of the engine. This means nearly but not completely full throttle, and 2000-2500 RPM. A smaller engine needs to be operated at closer to full throttle and at higher RPMs, meaning lower gears. This wastes fuel.

A good example: Toyota Yaris switched from 1.33 litre engine to 1.5 litre and the emissions and fuel consumption went down. The larger engine indeed was more economical!

Another good example: Toyota Prius used to be 1.5 litre but nowadays is 1.8 litre. Fuel consumption went down after switching to a larger engine.

Of course, this applies only to reasonably sized engines. If you want a 5 litre V8 in a passenger car, you shouldn't be expecting low consumption.

Answer 2

It's quite complex if you really get into the details of it but in a simplified sense your colleague is correct.

driving in the mountains

I feel this is a key point - driving in mountains means by definition that you are driving at altitude, as altitude increases the density of the air drops. This means for a given volume of air the engine takes in there's less oxygen available for combustion. Given the need to maintain a consistent (-ish) Air-Fuel ration this means that the engine will inject less fuel (no point in injecting more than you can burn!) per cycle so you get less power. This means you are going to be having to increase the RPM of the engine in order to do the same amount of work and this means you're going to be using more fuel (each RPM will come with the associated power "losses" of overcoming friction in the engine, accelerating the mass of the pistons and so on) just to get same acceleration from the engine.

A larger engine will generally be producing proportionally more power than a smaller one (all other things being equal) so even though it will be having to reduce fueling in the same way it's still going to be getting more power per revolution than the smaller engine and therefore won't have to increase that as much to get the same power as the smaller engine. And if the truck weighs the same as the smaller-engined one and you're trying to do the same speed then it will take the same amount of power, but potentially the larger engine could be producing that power more efficiently, and therefore having lower fuel consumption.

Answer 3

Some years ago, Top Gear stuck a Toyota Prius on the track and drove it flat out. Following just behind the Prius was a BMW M3, being driving in the wheel tracks of the Toyota, matching it's pace exactly.

The Toyota is a 1.5 litre hybrid, the M3 is a 3.2 litre straight 6 petrol. The Prius returned a fuel consumption figure of 17.2 UK Miles per Gallon, the M3 returned 19.4 UK Miles per Gallon. The reason given for this was that the Prius was being driven absolutely flat out, on it's limits. It was spending a significant amount of it's time with the throttle pedal mashed into the carpet. In order for the M3 to match the pace of the Toyota, it effectively had to be held back.

Now consider your example, a truck travelling up and down a mountainous region. When the truck is hauling a fully laden trailer up a steep incline, it will be using a significant amount of throttle. Climbing hills puts vehicle engines under strain and the two factors that affect this is the weight of the vehicle and the amount of power the engine is capable of producing. A small engine, worked hard will use fuel and generate heat at a quicker rate than a more powerful engine matching the pace of the smaller engined truck.

It is possible for a smaller engined vehicle to use less fuel than a more powerful counterpart but that's only on the basis that it takes longer to travel the distance.

Things like torque, (crank throw, conrod length), number of cylinders, design of cylinder head, efficiency and design of the exhaust system, etc, etc... all affect the fuel efficiency of a vehicle.

If every vehicle was equally efficient, manufacturers wouldn't publish fuel consumption figures and compete on efficiency and emissions.

Answer 4

To produce a fixed amount of power, with 100% efficiency, you have to burn a fixed amount of fuel per minute.

You can either burn it in a large number of small amounts in the cylinders of a small engine, or in a smaller number of large amounts in the cylinders of a big engine running at a lower RPM.

The amount of mechanical energy wasted in friction in the engine and transmission increases with higher RPM. For example if the friction force between the pistons and the cylinder block is approximately constant, if you double the RPM the pistons move twice as far in the same amount of time, and the same force does twice as much work.

So, at least within some practical range, a larger capacity engine in the same vehicle will tend to be more fuel efficient.

Note that the heat energy wasted as heat out of the tailpipe and in the engine cooling system depends more on the rate of fuel burn than the RPM of the engine, so the complete story is more complicated than the above.

All this logic breaks down if the engine size is so large that the "efficient" RPM would be impractically low. Also large engines weigh more than small engines, and need to produce more power to give the same rate of acceleration to the heavier vehicle. The effect of the weight increase is more significant for cars than for large trucks, of course.