Engine size/type (ie. 2L inline-4) and fuel consumption

1. Jun 8, 2009

StaticX

Hi everyone, new poster here. :)
Came across this forum the other day and found quite a few nice topics that were interesting and the people sounded like they knew their stuff ... so i joined.

Now the question...

I am interested in fuel efficiency as regards the engine capacity. For example, a 2L inline-4 versus a 6L V8. Generally in magazines or when shopping for new cars you see the stickers on the window saying it uses so many liters per 100km. So taking a typical japanese inline-4 2L you might see, say, 8L/100km fuel consumption. Whereas, for a 6L V8 you might see 13L/100km fuel consumption.

So what i want to know is this - why is it that a big engine like the 6L V8 consumes only a bit more fuel, like 1.5x to 1.7x, in comparison to a 2L inline-4? I mean the big engine weighs more, has 3x the capacity, often 2.5x to 3.5x the engine power of the smaller one, and 2x the number of cylinders so it obviously has 8 injectors squirting fuel into the cylinders versus only 4 injectors for the smaller engine. Sure the bigger engine often revs low in comparison but it has twice as many cylinders too and produces more power even at those low revs. I also think that for a petrol engine (in general) the minimum amount of fuel each injector squirts into the cylinder is the same proportional quantity with respect to cylinder volume (so for 2L inline-4 it would inject 1 unit into the 0.5L cylinder and for the 6L V8 it would inject 1.5 units into the 0.75L cylinder) as that is an inherent property of the combustion process to keep the pistons revolving (<-- is this last sentence clear as to what i mean??)

So i am basically puzzled as to how a much bigger and more powerful engine uses only marginally more fuel compared to the smaller one. So is the relationship not supposed to be proportional to engine size, number of cylinders, and power/torque produced? Can someone perhaps explain this and come up with a relationship that works well to predict fuel consumption with these variables in mind?

Thanks! :)

2. Jun 8, 2009

Ranger Mike

14 Feb 2009 post

Mgt3 is Offline:
good beginning

3. Jun 8, 2009

Ranger Mike

One more thing to consider
Starting in model year 2008, estimates reflect the effects of

Faster Speeds & Acceleration
Air Conditioner Use
Colder Outside Temperatures
EPA tests are designed reflect "typical" driving conditions and driver behavior, but the following effect your MPG

How & Where You Drive
Vehicle Condition & Maintenance
Fuel Variations
Vehicle Variations
Engine Break-In

4. Jun 8, 2009

Bob S

13 liters per 100 km vs 8 liters per 100 km is not a marginal difference. It is 63% more fuel. Gasoline engines typically are most efficient (about 30%) when operating at about 80% of maximum torque and 30% of redline. By efficiency I mean fuel consumption rate per horsepower output. This is actually measured with a dynamometer on the engine, and the measurement is grams of fuel per kilowatt-hour output. A good engine might use 240 grams per kilowatt-hour. If you put either of these engines in the same car, you will find that the larger engine is running further away from the maximum efficiency point, and is therefore less efficient. See attached BSFC map for a 2.5 liter engine. Ask for one at your automobile retailer.

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5. Jun 9, 2009

Ranger Mike

Bob S you make an excellent point in the first part of your answer, thank you..however

Last edited: Jun 9, 2009
6. Jun 9, 2009

StaticX

Ranger Mike, i've read the article you suggested in the first post and a few linked off that one too. Quite interesting read. Some things i never even thought off before i read that as it never entered my mind, since my background is electronics engineering and not mechanical but i still am extremely interested in cars hence why i'm asking.

Bob S, well sure 68% more fuel is quite a bit extra consumption but not when you consider the things i said in my original post, ie. "the big engine weighs more, has 3x the capacity, often 2.5x to 3.5x the engine power of the smaller one, and 2x the number of cylinders". So based on the capacity and power merit alone, when the V8 is 3x the capacity of the inline-4 why doesn't it use 3x as much fuel, or based on power if it has about 2.5x to 3.5x the power why not 2.5x to 3.5x more fuel?

So from reading the article that Ranger Mike suggested, i gather the cylinder size has an impact on how fast, after the air/fuel mixture is ignited, some of the heat is lost to the walls of the chamber resulting in a loss of potential energy before it can all be converted into torque and hence power. The other is volumetric efficiency but more or less i would think that most engine manufacturers have got this down pretty well. Other factors also being the duration and amount of lift of the intake and exhaust valves, piston bore and stroke etc...

But i guess my question is still if the V8 is 3x the size and 3x the power of the inline-4 why doesn't it consume 3x the fuel?
If it consumes only 68% more fuel to generate 3x the power, it seems to me that the V8 makes better use of the fuel to generate way more power than the inline-4. So why is that?

Last edited: Jun 9, 2009
7. Jun 9, 2009

Ranger Mike

good well thought out points..the following is strictly my opinion
why is the fuel consumption not linear with the displacement?
first lets look at internal friction that must be overcome to produce HP...I think there is 30 to 50 HP used up to overcome these parasitic drag in a race engine before you get anything out the flywheel,,you lose another 100 to 150 hp to the rear wheels depending upon the transmission and differential..

there is a large amount of parasitic drag in any IC engine.the majority is piston ring related..consider the parasitic drag of the oil pump, water pump..pretty much same on 4 and 8 cyl block...the main bearing and rod bearing drag will be very close since they run on oil film..the main drag comes from valve spring to lifter to cam friction and piston ring drag...I suspect there may be a doubling of the drag.
now lets look at the formula Power = work / time or power = force x distance / time or better yet ...
Power = force x bore x stroke / RPM
assume the same amount of fuel (gasoline)
the variables are displacement ( bore and stroke and number of cylinders) and RPM being the other variable
let us limit the RPM to 3000 for this discussion
if you have a finite amount of fuel and unlimited air
you get more power by spreading the fuel air mixture over 8 cylinders vs. 4 cylinders..but to keep the engine from leaning out..you have to change the fuel air ration to maintain 12 to 14 to 1 fuel air ration or you start to detonate or burn valves
the slight additional fuel is required to do this..this is not a doubling of the fuel air ration and hence the non linearity of the MPG
any of you degreed engineers ..HELP..jump in here ifin I am off beam..

8. Jun 9, 2009

Bob S

Another drag is running an IC engine at high manifold vacuum. This is work the the pistons have to do against the ambient air pressure on the intake stroke. Adiabatic expansion of the air under these conditions cools the air, which adversely affects the engine performance. This is why the torque for most efficient engine performance is about 80% of max torque, at any RPM.
Power = torque x (2 pi RPM/60)
torque = BMEP V /4 pi
So power = (BMEP V /4 pi) x (2 pi RPM/60)

where BMEP = brake mean effective pressure (in Pascals)
V = displacement (in cubic meters) (= liters/1000)
torque is in Newton-meters
power is in watts
Example
BMEP=10 atm = 106 Pascals
V=2.7 liters = 2.7 x 10-3 m3
RPM = 4000
so torque = 215 N m
Power = 90,000 watts = 121 HP
Compare to the BSFC map on my earlier post.

Last edited: Jun 9, 2009
9. Jun 9, 2009

chroot

Staff Emeritus
The amount of fuel the engine uses is not linearly related to its size, or number of cylinders. If you're cruising down the road at a steady speed, the engine uses exactly as much fuel as is necessary to exactly balance all the friction and drag forces.

A bigger engine has more parasitic friction, and its added weight creates more rolling resistance, so it will need to consume more fuel than a smaller engine. It might even be reasonable to expect a V8 to have twice as much parasitic friction as an L4, but other forces, like wind resistance (drag), are more important.

Imagine that a car with an L4 expends 25% of its fuel energy in overcoming parasitic friction in its drivetrain, and the other 75% in overcoming aerodynamic drag. If you kept the car body the same, but doubled the size of the engine, you wouldn't double its fuel consumption -- the biggest energy sink, aerodynamic drag, has not been changed at all.

- Warren

10. Jun 16, 2009

StaticX

Okay guys, i think i get the picture. The pieces are falling into place.

So in essence a bigger engine, in comparison to the smaller one, uses more fuel to get going from a standing start as it has more parasitic friction to overcome in the engine and mechanicals itself. But once it's going then it uses little fuel to support the cruising speed unless wanting to accelerate quickly thereby using a lot of power and hence fuel. The other thing is due to air resistance and drag which, whether you have a car with a big or small engine, requires a certain amount of power to overcome to get up to cruising speed.

So say the smaller engine has 100kW and the bigger one has 300kW. In this case, the L4 might use 30kW (im guessing here!!) to overcome the parasitic friction and drag to start accelerating comfortably whereas the V8 might use say 60kW due to more parasitic friction. But once both are cruising at a comfortable 80km/h then the L4 might need 60kW to sustain it whereas the V8 might need only 80kW (I was thinking here with respect to pressing the gas pedal, the RPM and the power produced, the parasitic friction which varies with engine, and air drag which i assume is same for both cars). Therefore, even though the V8 has so much power realistically you might only need a tiny fraction of the peak power output to do pretty much what a L4 does when driving normally around town and to the shops. So in effect, the L4 is quite suited to city driving with lower speeds of operation while the V8 is disadvantaged there but is though suited to highway driving where once it overcomes the higher parasitic friction doesn't require much power to sustain cruising speed.

So what i said above, is that pretty much kinda right? If that is it then i think i got a pretty good picture of the idea. :)

Thanks to all.

11. Jun 17, 2009

Ranger Mike

In my opinion..you got a pretty good grasp of it...the V8 will not necessarily have double the friction / parasitic drag ofthe 4 cylinder,,and would not exactly use double the fuel..ti V8 will make the required torque to move tha car at a lower rpm and is loafing along in city driving, wasting fuel idleing at traffic lights, etc...once on the highway the V8 can motivate a lot better with moderately more fuel consumption unless you hot rod it..

the aero drag is huge..just stick your hand out the window at 60 MPH

good going!