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

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?

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

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.

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.

http://ecomodder.com/forum/attachmen...1&d=1215707342

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.

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.

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.

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

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.

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.

Makes me think of the top gear fuel economy experiment between a prius and a BMW? on a racetrack. If you want to cherrypick an example you can make any premise correct I guess.

...only 'works' when "...all other factors..." are equal, ie: weight, gearing, aerodynamics, drivers, etc.

I dont think the Prius was cheated in any of these, it was cheated in that it was run WOT around the track and the other car was not WOT to keep up cause it was significantly more powerfull.

I dont disagree that the top gear episode was somewhat stupid, the point they were going for was that the driver was the biggest component towards good FE, which it is. But it does also make the case that if you have to run an engine at WOT all the time then it definitely is to small to get optimum FE.

For those who have not seen it.
Start at 4:20
http://www.youtube.com/watch?v=JmxUsGiGp3w

Wide open throttle is pretty efficient, come on, you're an ecomodder :P

A Prius is extremely efficient even at maximum output, since it essentially has a CVT. The problem is that it was probably really struggling through turns and braking a lot while the M3 shrugged the corners off at the same speed.

In The boating world the 2 stroke direct injected outboards are reportedly getting better fuel efficeny than the same size larger four stroke, direct injection and no throttle body might be the ultimate answer.

I think the guys driving the Ecoboost trucks are using all that torque way too often, killing the fuel mileage, pretty strong motor,

One of us obviously doesn't understand BSFC charts. Is not efficiency represented by the contour lines? So in that chart, peak efficiency would come at about 2500 rpm and 124.7 ft-lb?

...yep. :thumbup:

...like in these two Prius engine examples:


http://ecomodder.com/wiki/images/5/5...rius_bsfc2.jpg

...and this chart:

http://www.libralato.co.uk/images/fig53.jpg

No its not because at WOT you are running the engine on fuel enrichment, you would want to run at the maximum throttle opening that permits stoichiometric air/fuel ratio. Maybe thats 80% or 90%, I really dont know the point, but WOT is not where its at for FE on a stock production gasoline engine.

I noticed, Anthony posted the link to the article on a gaming forum I moderate. I believe he's joined here under the name "RunningStrong". :D

I think the premise is good, but needs more research. Anthony knocked it together based on a conversation we were having on that forum about the Ford EcoBoosts.

In part, I agree... sometimes an engine is too small for the work desired. When I did the EcoBoost Explorer, fuel economy in traffic was just as bad as the V6. Whatever the size of the engine, accelerating two tons of metal (however gently) up to speed requires a lot of gas. But I partly disagree, because from my driving, I saw the potential for extra highway MPGs simply by staying out of the boost.

Unfortunately, since part of the premise of the EcoBoost is that you don't give up power for economy, these new engines are tuned to produce lots of boost very early, so most people don't see any benefits.

...the 1.4L turbocharger Cruze boost is all-in by 1,850 rpm...and idle is only 800 rpm!

You might be surprised how many cars do not enrich until high rpm. Fairly sure the Prius does not enrich the mix, ever, because its peak efficiency is pretty much at maximum torque output.

I was also going to note this. In most cars I've been in you don't hit fuel enrichment at WOT until maybe 4000+ rpm.

Not sure but specifically I think before wideband O2 was popular it was common, I know my 1ZZ-FE is closed loop into the 4000s, never actually logged it in open loop mode!

I think wideband O2 equipped cars are running richer at max torque to reduce exhaust temperature a little bit and make the cats last longer. I followed the BRZ/FRS pretty closely and the AFR goes down to 10.9:1 at high rpm! It dips into the 12s well before 4000. Reflashing that to 13:1ish gains a huge amount of power...