diesel fuel usage question 6

[IceStationZebra]

I have been tossing this around for a couple of days. Let me know what you think/know.

Assume you had two identical vehicles (mass, air resistance, tire size, ground clearance, cooling fan, radiator, thermal efficiency, etc) except one had a 6L v8 diesel and the other had a 4.5L v8 diesel. Assuming that neither engine has reached maximum power while driving at a constant speed would they get nearly identical fuel millage? My reasoning is that since diesels only provide the fuel/power you need wouldn't they both use nearly the same gal/hr?

The only real-life example I can think of is two semi trucks with two different Hp rated engines, but I don't have any experience in this area.

ISZ

 

[Lou Scannon]

If both engines were optimized for fuel economy for the mission in question I think there would be very little difference. In the practical world, as long as the 4.5L could perform the mission in question (including longevity), it would win commercially, since it would tend to cost less to manufacture, and would permit marginal reductions in the vehicle structure.

 

[GregLocock]

Why not look at the bsfc map and decide?

For instance, if this is a good model

http://www.tc.gc.ca/programs/enviro...nology/study2/Final_report/image/Final_27.gif

Then the smaller engine could well be operating at a better bsfc.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

The bigger engine will be heavier, will have more losses due to internal inertia, will have more friction, will pump more air and will have higher surface area to reduce thermal efficiency.

It will also make the same power at lower rpm, so it will have more torque, thereby requiring a heavier drive train which will also absorb some more power, so vehicle tare weight and fuel consumption will be higher.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[JSteve2]

The correct answer is the properly sized engine will get better fuel economy - big engines for heavy vehicles and small engines for light vehicles. In your problem, neither engine near maximum power suggests that the smaller engine will get improved fuel economy for the reasons stated above. The only reason that would not be the case is if the smaller engine had to run non-optimal combustion to keep up. For example, if the small engine were running near max torque or in some part of the speed-torque map where it had to run an exotic emissions routine, and the big engine was in the sweet spot, or the small engine turbocharger was forced to run in an inefficient spot of the compressor map.

 

[MikeHalloran]

I've commuted with otherwise similar Camaros running V6 and V8 SI engines. Within the precision of my measurements, they got the same fuel mileage when using the same driving cycle, i.e. keeping up with the same gang of idiots every day for a couple of years.

But V6 and V8 Camaros are more similar than are the sort of trucks in which you might find 4.5 and 6.5l Diesels.






Mike Halloran
Pembroke Pines, FL, USA

 

[IceStationZebra]

This question applies to many applications not just cars & trucks. I guess a more straight forward way to phrase the question would be:
If both a 6L and 4.5L v8 diesel engine were run at a constant 1100rpm with a 70hp load, would the fuel usage be nearly equal?

- I am using less than full power as an example because that is how most of the engines I deal with operate 99% of the time, which includes cars & trucks.
- I realize that in auto/truck applications the bigger engine would almost certainly need a larger/heavier drivetrain and would have different gearing. Of course this complicates a direct comparison.
- JSteve2 - I hadn't thought of the turbo efficiency slant. Thats a good point! This goes right along with Greg's efficiency map idea.
- Pat - I had considered the friction/pumping losses. I was thinking that if both are v8s at the same speed the losses would be similar, but obviously not equal. Safe assumption?
- Mike - I assume that the v8 was turning slower at a given road speed, otherwise the fuel usage would have to be higher. Running a SI engine at constant speed requires a certain amount of fuel to keep the fuel/air ratio correct even if the load changes, whereas a diesel only has to inject what's demanded to satisfy the load.

ISZ

 

[patprimmer]

On a diesel, pumping losses will pretty much be directly proportional to displacment times RPM.

Inertia losses in the reciprocating parts will be proportional to maximum piston speed times rpm times piston weight with speed being much more significant according to the famous Mr Newton. force = 1/2 M x V x V

Friction is more complex as cylinder pressure influences the ring friction. The bigger engine will have more piston travel and/or greater ring to bore contact length, but lower cylinder pressure at a given power level.

The bigger engine will still require a heavier duty gear box as it will be capable of a higher peak torque, presuming similar boost levels. The designer will need to design the drivetrain to maximum, not typical torque output.

If an engine is 20% bigger, it won't use 20% more fuel, but I expect it will use enough extra to be measurable with well controlled tests and careful measurements with accurate instruments.



Regards

eng-tips, by professional engineers for professional engineers
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[JSteve2]

ISZ,
If your question is whether diesels are much more robust to being "mis-sized" then that is true. That is part of the reason diesel-hybrids have not been popular. Among other efficiency gains, hybrids allow you to use a mis-sized engine, having lots of power when you need it, but shutting it down when you don't.

 

[SomptingGuy]

Diesel hybrids are VERY popuar in the rail sector here in the UK (where electrification is poor). I just love it (because I'm sad) when you can hear a big diesel engine switching between its two main operating modes: peak power vs peak efficiency.

- Steve

 

[IceStationZebra]

Thank you for your answers. I was trying to make this a pure engine question, as the answer has several implications to my areas of interest. I list just two examples below.

Example 1 - In the off-road machinery industry it is not uncommon to have multiple Hp/kW choices for what is basically the same machine. And the engine choices are usually all the same brand/displacement/aspiration. A simple example would be a skidsteer loader. Obviously the larger engine CAN use more fuel, but if it isn't working very hard will it? My hunch, which was mostly supported by the answers above, is probably a little more to make up for the extra friction/pumping looses.

Example 2 - GM's choice to make the new DuraMax a 4.5L affair. I was hoping that it would be smaller to get greater efficiencies, or maybe have a little brother?

http://www.eng-tips.com/viewthread.cfm?qid=206747

ISZ

 

[dgallup]

Your Ex 1 is quite a different situation than your original post. If you are considering the same block & same displacement diesel with just a different power rating then there are far fewer variables. I worked in a major truck manufactures engine development laboratory right out of collage. We made 2 basic blocks, a straight 6 & a V8. Both engines could be had in multiple power ratings. In every case the highest power rating would have the best BSFC in a small "island" of speed and load. However, if you compared equal power/speed BSFC the lower rated engine would be marginally better because the turbo & injection would be better matched for those conditions. The differences were not huge but measurable under controlled testing.

 

[truck]

Being a northerner, I think there is one consideration you miss: if you would like to have warm toes, you'll be a lot happier with the smaller engine.

Truck

 

[TDIMeister]

"This question applies to many applications not just cars & trucks. I guess a more straight forward way to phrase the question would be:
If both a 6L and 4.5L v8 diesel engine were run at a constant 1100rpm with a 70hp load, would the fuel usage be nearly equal?"

The only way to tell for sure it to look at the BSFC maps for both engines. However, given the same technology, cylinder count, layout, geometrical similarily (e.g. mean piston speed), etc., the larger displacement engine will tend to have a lower FMEP. However, since FMEP is a function of displacement, it is wrong to conclude that gross friction will be lower. On the contrary, in the example above, friction will be higher for the larger engine.

However, what has already been said about turbocharger operating points, the influence of emission-control strategies (e.g. EGR rates), and injection timing to add one more all play roles, so the answer is not always clear cut. But in the case of the example given above, the smaller engine will quite confidently have lower fuel consumption, all other factors remaining the same.

 

[SonyAD]

Patprimmer gave an excellent answer.

Stroke and crank throw would also matter for fuel efficiency. Besides differences in reciprocating masses the engine with longer stroke will have its pistons reach higher linear velocity inside the cylinder bores for the same rpm even with the same bore to stroke ratio and stroke to crank throw ratio.