Big engines and mpg

[Stratman]

The current thread on the C55 consumption has revived one of the questions that has had me puzzled for ages.

Take two cars of the same model but with different engines, say a C180 and a C55. Have them drive side by side at 70mph on a motorway. They are not accelerating, they are both displacing the same amount of air (I'll ignore the very small difference caused by the wider tyres on the C55) and the rolling resistances are close but not identical. To a first approximation they are producing the same power, since we have the same bodyshells doing the same steady speed.

So why will the C180 be getting around 40+ mpg and the C55 around 30 or less?

 

[This is Pete]

Good question. The C55 has is 5.4 litres, the C180 is 1796cc

More volume, and more petrol to spurt into more cylinders per cycle.

That's my theory at any rate.

Spurt: My new favourite word of the day.

 

[Wasabi]

C55 is heavier.

 

[This is Pete]

C55 is heavier.

By 200kg, so not a massive difference. When rolling at a constant speed, it shouldn't cause that much of a difference.

 

[Stratman]

Pete, the question is why the bigger engine has to squirt more petrol to make the same power.

Wasabi, when the cars are doing a steady speed the weight doesn't matter except for calculating the (small) difference in rolling resistance.

 

[This is Pete]

I'm no mechanic, but I assume that larger cylinders would need more petrol to make them cycle with any degree of efficiently.

A quick squirt of Fairy Liquid will wash your dishes in the kitchen sink, but the same amount in your bath won't get you clean.

 

[SavMan]

You will need one of the engineers to give you an accurate explanation but I have had this conversation before with the guys that work on my Skyline. From memory there is firstly rotational mass with the bigger heavier engine requiring more energy to rotate than a smaller and lighter one, the larger engine will also have greater surface area so will have higher frictional loss. Secondly in order to create an explosion the larger engined car will typically need more fuel as it will also have more air due to greater flow and larger cylinders etc Third the larger engine is typically running at a lower rpm and at normal driving condition is below its optimum efficiency at some point this starts to even out so for instance at 125mph the smaller engine will typically be running at its max air fuel ratio. Finally and this is where I really get lost it is something to do with thermodynamics but my GCSE Physics was to long ago for me to be able to help with that.

 

[This is Pete]

We need someone to give James May a call...

 

[Peter DLM]

From what I understand, from my motorcycle racing/tuning days, SavMan is pretty darn close to the explanation. We spent a lot of time and money reducing rotating mass and frictional losses.

 

[Deleted member 65149]

... at 70mph on a motorway .... the C180 be getting around 40+ mpg and the C55 around 30 or less

A few guesses at an answer, but is the question correct? Is there really that much difference?

I wonder if both cars were held at 70 by cruise control, would there would be much less difference? The C55 would be working less to maintain the speed, but a lesser experienced right foot may keep lots of extra fuel flowing unnecessarily. I've found that the indicated instant fuel consumption can drop quite significantly on a larger engined car by easing off on the throttle without any drop in speed. It's even more pronounced in my new C350 with its low rolling resistance tyres. Has anyone else noticed this?

And could this be why so many people report not being able to achieve anything like the manufacturer's MPG figures? Simply a heavier-than-needed foot?

 

[A210AMG]

Your right even in my older S211..

Easing off the throttle while still maintaining the same speed does seem to give more mpg. However on mpst roads that will allow I use the cruise control. I guess as my mother always puts it 'feathering lighty' would give more mpg.

 

[Deleted member 37751]

It would also be good to see at what speed the mpg figures match, I would hazard a guess of 90 mph.

 

[whitenemesis]

" In thermodynamic terms, bigger engines are more efficient. But if too large an engine is specified for the average duty cycle being asked of it, you won't be running as close to peak efficiency as a smaller engine, and a smaller engine will outperform in fuel economy."

Why are big engines so horribly inefficient? [Archive] - Physics Forums

I'm guessing if the cars had gearing to bring their engines both into their most efficient rev range the differences would be insignificant?

 

[Bellow]

To understand it you have to look at where the 60+% of fuel is going, leaving you with approx 30-35% efficiency.

Firstly - air pumping losses. For the same power output (to maintain 70mph cruise) the small engine is running with a larger throttle opening (load factor). Throttling losses are significant on petrol engines and the nearer to wide open throttle (WOT), the more efficient per hp produced is the petrol engine.

Secondly - heat loss. The large internal surface areas of the larger engine present a greater opportunity for the combustion heat to leach away to the coolant. This is compounded by a larger engine typically running slightly slower as the time for heat loss becomes greater.

The above two reasons are why cylinder de-activation is becoming popular.

Thirdly - frictional losses. This is the killer for large engines at part load especially, and is obvious when comparing say, a 2.0 litre 4cyl with a 4.0 litre V8. The friction is doubled. Friction is largely independent of load factor so assumes a significant part at cruise conditions. Cylinder de-activation is of limited benefit to friction as the pistons are still moving but contributing nothing. Much more promising is piston de-activation where redundent pistons are parked until required. There is also viscous friction caused by pumping the oil and by the shear forces within the oil films in the bearings and especialy between the pistons and cylinder bores.

There are also other factors such as compression ratio which is easier made higher on a smaller cylinder - but then the surface/volume ratio suffers...

The effects of the main losses highlighted above vary over the operating range, both with speed and load, sometimes non-linearly, sometimes in concert, often contradictary - eg higher rpm gives less heat loss but greater friction. This highlights the following.
That the engine be designed/selected for the type of running it will most often encounter. That the driver operates it accordingly.

With absolute certainty it can be said that per hp delivered, the petrol engine will deliver its greatest efficiency when operated at maximum torque output. This point represents best breathing (where the air work (pumping losses) are at a minimum), and the best trade off of frictional losses versus heat loss is achieved. An approximation of that can be achieved at lesser rpm as, say, half throttle is effectively full throttle at half the rpm of maximum torque - hence the requirement of low engine speed and highish (relative) load factor for economical cruise. That there is little surplus power available during such cruise conditions is indicitave of it effectively running at WOT. And that is where the flexibility of the transmission comes into play....

 

[DrNick]

So does this mean that we get best efficiency (and best fuel economy) by accelerating hard to a high speed and then coasting down to a low speed, rather than maintaining a constant speed?

 

[Stratman]

Thanks one and all for the brain power.

I sort of guessed it would be something to do with the mechanics of the engine itself, but was half expecting one 'big' factor. It would seem that there are lots of little reasons which on their own could almost be discounted (like the extra air resistance of the wider tyres), but when added together make the difference.

I think the following quote from the link in whitenemesis' post is the clincher:-

For heat engines in general, the theoretical limit of efficiency is determined by the difference in temperature between the hottest point and the coolest point in the thermodynamic cycle. In the case of a gasoline engine the compression ratio is the main limit. But running the engine "throttled" means that the entire cycle will be less efficient. (search for otto cycle) A big engine is severely throttled almost all the time, hence it's inefficient.

Keep 'em coming. It's a fascinating subject

 

[Colin_b]

Look at the new Focus, very small engine. 1 Litre, 3cyl and minimised internal friction. Very efficient at light loads, with lots of turbo boost for power.

100 or 120bhp, upto 200nm torque.

Don't know how practical it will be in durability terms - though 120bhp/litre isn't exceptional.

 

[Palfrem]

What is the engine RPM at a given speed?

 

[Dieselman]

SAvman, Peter DLM and Bellow are indeed right on the money, apart from Bellow's 30-35% efficiency, should read 25% at best, much less on part throttle.

To understand it you have to look at where the 60+% of fuel is going, leaving you with approx 30-35% efficiency.

Firstly - air pumping losses. For the same power output (to maintain 70mph cruise) the small engine is running with a larger throttle opening (load factor). Throttling losses are significant on petrol engines and the nearer to wide open throttle (WOT), the more efficient per hp produced is the petrol engine.

 

[Bellow]

So does this mean that we get best efficiency (and best fuel economy) by accelerating hard to a high speed and then coasting down to a low speed, rather than maintaining a constant speed?

You already do this to some extent and to a greater degree than you might realise....
To pick up in your particular vein, you appear to be asking 'why not hoof it up the road for all it's worth?' (then coastt). Fine, but you are going to use more power and therefor fuel than is necessary to hit a speed from which to coast down from. (To maintain the same average speed would require a higher peak speed be attained prior to lifting off, an overshoot, if you like - so a steady cruise makes more sense).
That still leaves you to decide the level of acceleration to use to attain the cruising speed while avoiding running the engine overly throttled.
Simple - once you get your head around the 'half throttle is full throttle at half revs' conundrum.

Maximum torque represents the maximum volumetric efficiency (the 'fullest' the cylinders can be) at the highest rpm (high volumetric efficiencies can be obtained at lower than peak torque speed). Beyond the speed of maximum torque the induction system becomes a throttle in itself, restricts breathing, and causes the cylinder to fill to a lesser extent than before (the continuing rise in power is because although each firing event provides less punch - there are more of them. Untill the speed of maximum power is reached which is where the falling volumetric efficiency combines with the rising friction to call a halt to proceedings).

So, if the engine produces maximum torque at say, 4000rpm, then what that means is that beyond 4000rpm at any throttle opening, it will be throttled and operating at a lesser efficiency.
Conversely, up to 4000rpm, wide open throttle (WOT) means 100% unthrottled. And as WOT can flow enough air for 4000rpm, then half throttle is good enough for unthrottled running up to 2000rpm. (Beyond 2000rpm at half throttle the engine cannot get the air required to sustain a good volumetric efficiency and the power levels off - which is the point at which a driver of a manual transmission will change up a gear).

Driving through the gears as per the above paragraph will give good economy as the engine is running unthrottled, the frictional losses (related to rpm) are moderate, and the fuel consumed in rate of acceleration is kept reasonable as each gear change to access the most efficient part of the engine's operating regime also sees an increase in the overall gearing which becomes an effective torque limiter to acceleration. Thus we have removed the need to limit the acceleration by throttling and have avoided wasteful acceleration which by its magnitude would be consumptive of fuel, aided and abetted by the lowering of engine efficiency incurred by using higher rpm.