Are Smaller Engines More Fuel Efficient than Bigger Ones?

[maxvan22]

This is a question I was thinking about all the time, because smaller engines use less fuel but need to work harder, and bigger engines use more fuel but need to work less.

For the test I used a Mercedes-Benz Actros Megaspace.
Chassis: 4X2
Gearbox: Powershift G281-12 (same for every test)
Fuel Tank: 600 Liters

I used the stock gearbox because it had a final ratio of 1 so the test would purely be the performance of the engine

NOTE: these figures are from the cab's consumption screen, and for every test I was pulling a 9 metric ton load, and I only recorded the figures at 90kph in 12th gear with cruise control on. I also had realistic fuel consumption on.

For the first test I used the smallest engine that had 320HP and 1650Nm at !,080rpm. Below are the averages.

-When on a flat road surface the engine was consuming 36.7 liters for every 100km
-On a slight incline the engine was consuming 43.5 liters for every 100km.
-And on a downhill the engine was consuming 0.0 liters for every 100km.
-The average was 39.2 liters for every 100km


For the next test I fitted the truck with the most powerfull engine that produced 598hp and 2,800Nm at 1,080 rpm. Below are the averages.

-When on a flat road surface the engine was consuming 39.1 liters for every 100km
-On a slight incline the engine was consuming 42.2 liters for every 100km.
-And on a downhill the engine was consuming 0.0 liters for every 100km.
-The average was 38.6 liters for every 100km

Conclusion: With a light load and a gearbox with a final ratio of 1 the bigger engine worked less, but on a level surface the bigger engine consumes more.
Over all the bigger engine is better.

[maxvan22]

If anyone has any questions I will be happy to answer them

[Hypertext Eye]

I have some comments/questions about your methodology.

I used the stock gearbox because it had a final ratio of 1 so the test would purely be the performance of the engine

It shouldn't matter which gearbox you use, as long as you use the same gearbox for every test. You're just testing engines. As long as the engine is the only variable, you should be fine.
You could do similar tests later where you pick an engine and see how different gearboxes affect things, that might be interesting too. Actually if I were doing the testing I might have even done that first.

Did you measure on the same stretch of road?
Reset the trip meter before each test?
You mention the load mass was consistent, but was it the same trailer also? How did you manage it? I would have picked up a load, drove to the upgrade shop, and made a manual save, to make it quick and easy to test different parts. Also using the dev console/camera to teleport to the testing site.
Did you start from 0 or wait until you got up to speed before collecting data? Acceleration does play into fuel efficiency.
Why did you choose 90 kmh as your cruise speed? I would have thought 80 would be more appropriate, it seems the more common speed limit (I could be wrong).
I would like to see data with cruise speeds of 60, 70, and 80.
Also, more engines! Not just the high end and the low end.
Flat vs. incline vs. downhill: all the same length? If flat is longer then average will favor the smaller engine, if incline is longer then average will favor the larger engine. (it seems downhill is irrelevant, but that's expected)

It occurs to me, the best way to test fuel efficiency might be with a custom map mod made just for that purpose. Also you can use the "warp" console command to slow down the simulation, make it easier to end the test precisely at 100 km driven.

I'm sure there's more I haven't thought of yet.

ETA: I'm sorry if it seems like I'm picking on you now. You seem to be pretty serious about this so I just want to help you improve your test methodology.

[maxvan22]

Originally posted by ハイパーテクスト・アイ:

I have some comments/questions about your methodology.

(Didn’t want to quote the whole thing because it will take up to much space)

I did the same load with my trailer.

Yes. I did reset the the fuel consumption meter by stopping and waiting for it to go down before trying it again.

As it says, I only recorded data once I was at 90kph and in 12th gear

I choose 90 KPH because it will be easier to do the math, and because its the normal highway speed, yes it is sometimes 80 KPH, but its 90 most of the time depending where you are.

I could include more engines but most of them would just have a difference of 0.1 or 0.2 litres.

I didn’t use a custom map mod because the piece of road I was using was straight with a small incline.


P.S. not trying to be rude, but you sound like my old physics teacher lol.

“This is great, but...”

[Hypertext Eye]

Originally posted by mark:

Yes. I did reset the the fuel consumption meter by stopping and waiting for it to go down before trying it again.

That's not the same as what I was asking about. There is a trip meter that tracks distance and fuel usage. You assign a button that resets the trip meter to 0. Reset the trip meter before every trip, and use it. Otherwise I can't trust your data. Trip meters are designed exactly for this purpose, to check fuel efficiency in real life situations. You should be using it for your tests. If you only want to test for steady state velocity: turn off traffic, accelerate to cruise speed, reset the trip meter, drive 100 km on a straight and flat road, pause the game and record the data. Then repeat for various grades of incline.

Testing steady-state velocity is fine but you should also test for acceleration. It's a significant portion of fuel usage, especially in stop/start situations (cities). I have thoughts about how to do that, too.

I choose 90 KPH because it will be easier to do the math, and because its the normal highway speed, yes it is sometimes 80 KPH, but its 90 most of the time depending where you are.

Yes, that's exactly the problem. It depends where you are, which is why we need data for various speeds.

Even if the engines are so close, more data is still useful. At least include an engine in the middle. Most people IRL aren't driving with the weakest or the strongest engine.

I didn’t use a custom map mod because the piece of road I was using was straight with a small incline.

Ok, are you serious about this or not? lol
I realize it's a lot of work, but we don't do things halfway around here. (or maybe we do? lol)

P.S. not trying to be rude, but you sound like my old physics teacher lol.

I'm a chemist. It's not surprising.

ETA: If you want me to leave you alone, just say the word. I won't bother you again.

[maxvan22]

I was just seeing if bigger engines where better in a highway scenario.

You’ve given me some great ideas but, I didn’t want to go in that much depth like I did for my most fuel-efficient truck. I just wanted to give a simple answer to a simple question, and if you would like to find out the fuel efficiency of medium sized engines it’s quite easy and you can do it yourself.

Thanks for the feed back, I greatly appreciate it

[Hypertext Eye]

Originally posted by mark:

I just wanted to give a simple answer to a simple question

"Simple" questions are often not so simple as they first appear
Anyway you do you, man. If you're satisfied with your answer, that's all that matters.

[crazy rocker]

depends on the cargo ur hauling. gearboxes impact ur fuel more than engines do. heavy load u need better gearbox with better ratios to pull away on steep hills but for lighter loads a 6 speed gearbox is more efficent than 12 or 12+2.

In the real world small engine cars r better around towns but bad on motorways. IE my sister had a Nissan micra 1 litre. i had a SAAB 9000 2 litre ecopower turbo. we did a 70 mile round trip to the beach in r own cars. her fuel bill came to £20 mine was £5 cause we used a motorway. (she had too many kids to take them all in her car so i took some in mine)
round town stop start driving my SAAB was doing 17MPG on a long run at a steady 70MPH it was getting 70MPG. the Micra was getting 25MPG around town but only 10MPG on motorways.

[Hypertext Eye]

Originally posted by crazy rocker:

depends on the cargo ur hauling. gearboxes impact ur fuel more than engines do. heavy load u need better gearbox with better ratios to pull away on steep hills but for lighter loads a 6 speed gearbox is more efficent than 12 or 12+2.

You completely missed the point of the discussion. Way to go!

[samcan244(73 61 6d)]

Originally posted by crazy rocker:

depends on the cargo ur hauling. gearboxes impact ur fuel more than engines do. heavy load u need better gearbox with better ratios to pull away on steep hills but for lighter loads a 6 speed gearbox is more efficent than 12 or 12+2.

In the real world small engine cars r better around towns but bad on motorways. IE my sister had a Nissan micra 1 litre. i had a SAAB 9000 2 litre ecopower turbo. we did a 70 mile round trip to the beach in r own cars. her fuel bill came to £20 mine was £5 cause we used a motorway. (she had too many kids to take them all in her car so i took some in mine)
round town stop start driving my SAAB was doing 17MPG on a long run at a steady 70MPH it was getting 70MPG. the Micra was getting 25MPG around town but only 10MPG on motorways.

yea engines are most efficecnt when under high load which means fully open throttle and low RPMs tiny engines have... trouble shall we say with higher speed cruising