GCR: is it possible to make MPG worse by downsizing engines TOO much?

[MetroMPG]

Today, Green Car Reports posted what at first glance looked like a potentially interesting article:

Getting Good Gas Mileage Is Hard; Will Tinier Engines Really Help?

The premise of the article is that old chestnut (paraphrasing):

"A smaller engine may not be good for MPG because you have to drive it much harder than a bigger engine."

For dear reader's consideration, writer Antony Ingram offers up various bits of evidence:

• "real world" results of the UK's Focus 1.0L EcoBoost and the Fiat 500 2-cylinder cars are only barely better (or even equal) to the "real world" results for the same car with larger engines
• He claims Fuelly shows that owners of the Ford F-150 V6 Ecoboost are getting the same 17 MPG average as V8 F-150 owners (I challenge this in the comments - not sure where he's seeing these numbers)
• the latest Prius has a better EPA rating & real-world results even though it's using a larger (1.8L) engine than the previous generation (1.5L)
• and a couple of other points...

I think the article starts out asking a really interesting question, but in the end I think he's a bit too sloppy with the "evidence" and he wrecks it. (In my opinion.)

I couldn't help but comment. Then comment again... and again.

Getting Good Gas Mileage Is Hard; Will Tinier Engines Really Help?

[user removed]

When you configure a smaller displacement engine to provide both power and economy then the driver has the ability to improve mileage if they change their driving habits to increase overall efficiency.
If they drive the same way as they did with the larger displacement engine then any gains are probably negligible. I would love to see what the 1 liter engine would do in my Fiesta, but that will probably be a long time since it will have to be a rebuilt 2014 Fiesta. I would guess that I could average 50 MPG versus my present 45.5, maybe better.

regards
Mech

[PaleMelanesian]

It takes fuel to make power. What these downsized-and-turbo engines give you is the option to choose how much power you want. Along with that comes the ability to choose fuel consumption. Drive lightly and they'll reward you. Drive with a heavy foot and you'll get the mileage to match.

In the F150 Ecoboost comparison, you have to consider that the V6 turbo is capable of more power and torque than the V8. If you actually use that power, you'll necessarily consume more fuel as well. The beauty of it is, if/when you don't use it, you save fuel compared to loafing along with the bigger engine.

[shovel]

Originally Posted by PaleMelanesian

It takes fuel to make power. What these downsized-and-turbo engines give you is the option to choose how much power you want. Along with that comes the ability to choose fuel consumption. Drive lightly and they'll reward you. Drive with a heavy foot and you'll get the mileage to match.

In the F150 Ecoboost comparison, you have to consider that the V6 turbo is capable of more power and torque than the V8. If you actually use that power, you'll necessarily consume more fuel as well. The beauty of it is, if/when you don't use it, you save fuel compared to loafing along with the bigger engine.

From my experience and understanding, I agree with the above.

No matter how efficient you happen to make an engine, there is still work to do in accelerating mass and pushing an object through air. Even if we refined the internal combustion engine to its theoretical limits of perfection, the efficiency of the vehicle as a whole would still come down to mass, aero, and driver input.

Because they're tuned for efficiency at cruise, I don't think any engine to date returns good efficiency when it's being flogged - so if you need to flog it constantly to achieve the momentum you expect, it seems only natural that's going to be less efficient.

[Daox]

BSFC charts show that an engine under high load runs more efficient than an engine at low load. This is why smaller engines work more efficiently. You can not 'work an engine too hard'. If we look at a BSFC chart you can see why.



The top line is peak torque output. That is what you get when you are at WOT (wide open throttle). Along that line, worst case scenario in the above map is at 6000 rpm (cause nobody drives around at 750 rpm). So, even if you are beating the living snot out of your engine at WOT and peak rpms, you are likely still more efficient than normal cruising, and especially so with a larger engine.

For example, in this case lets say the car is crusing at 65 mph and uses 14 hp to do that (I went through this example a while back here). The red points are where the engine would operate at different rpms (different gearing). If we had a smaller engine, all those points would be shifted upwards which means higher engine loads. That also means the engine would operate more efficiently. A larger engine would shift them all downwards resulting in the opposite.

You can see it is theoretically possible to operate the engine at such high loads that you SLIGHTLY miss peak efficiency. This means driving around at WOT ALL THE TIME. This simply does not happen.

[oil pan 4]

Back in 1982-86 GM tried putting their pathetic 2.8L V6 in blazers and jimmies.
The result was a 16/18MPG and you couldn't tow anything. A 2mpg savings over the gas 5.0L V8 and equal MPGs to a 6.2L diesel.

[wungun]

High load I can understand...
But high rpm isn't that efficient because of the frictional and pumping losses...

What ever happened to the Orbital engine?!
Turbo charged, direct injected 2 stroke. Clean, small, REALLY Powerful...
And less frictional losses (4 strokes have 3 strokes that have to be "pushed" by the other cylinders. Or large heavy flywheel.
2 stroke, we'll everytime the piston goes down is a powerstroke!
It was all the buzz years ago...and a lot of the major manufacturers were experimenting with the concept.

[ksa8907]

I can say that the transmission plays a big role. My mother in laws dodge journey has a 2.4 4and cylinder with 4 speed auto and going up mountains on our way back from Tennessee, I had it floored and we were losing speed at 70mph. Engine was around 6k rpm

[Daox]

Originally Posted by wungun

High load I can understand...
But high rpm isn't that efficient because of the frictional and pumping losses...

It is that efficient, the map shows the efficiency through the whole rpm band. It shows the efficiency drop due to frictional losses.


Gearing is definitely a big deal in the efficiency of the vehicle. However, my point was that you can not 'work an engine too hard'. Thus, the idea of having too small an engine for fuel efficiency reasons is mute. Obviously there is a point at which it becomes too small to be practical, but that was not the intent of the article.

[oil pan 4]

Originally Posted by wungun

Turbo charged, direct injected 2 stroke. Clean, small, REALLY Powerful...
And less frictional losses (4 strokes have 3 strokes that have to be "pushed" by the other cylinders. Or large heavy flywheel.
2 stroke, we'll everytime the piston goes down is a powerstroke!
It was all the buzz years ago...and a lot of the major manufacturers were experimenting with the concept.

2 strokes always had problems meeting emmissions requirements.
That is why detroit diesel abandoned the 2 stroke diesels so many years ago.
Plus the big old detroits needed a blower to move air throught the cylinders.