Turbo vs. N/A city fuel economy? 2

[drwebb]

Some passenger cars offer gasoline engine options for the same displacement but with a higher compression ratio normally aspirated- and a lower CR turbo mill. The turbo has higher max power output of course, but I am wondering the likelihood it's lower CR will lead to higher city fuel consumption (where the turbo isn't boosting as much) compared to the gas model?

As far as I can tell from EPA mileage ratings (for what they are worth) the mileages are the same.

 

[Lou Scannon]

If the EPA economies are the same it's unlikely there would be a significant difference between the two in the same, normal use. However, if one of the versions inspires a different (i.e. more spirited) driving style than the other, that would most likely show up as a difference in economy.

 

[BrianPetersen]

I don't think there are many cases where the engine is exactly the same, in North America.

For a specific example, go to this link and select an engine:

http://www.vw.co.uk/new_cars/golf/engines

If you want to compare gasoline engines, unfortunately the 2.0 turbo FSI engine that we get in North America doesn't seem to be available there, and the 2.0 non-turbo FSI available there is not available here, but maybe further poking on the website might find something with the 2.0 TFSI over there.

For diesels it's easier in that particular case, the "S" 2.0 SDI is the non-turbo version of a TDI, 6.8 L/100 km urban and 4.7 L/100 km extra-urban. The "Sport" 2.0 TDI is the same engine with turbo (and almost twice the power), the fuel consumption ratings are exactly the same.

 

[Marquis]

When comparing vehicles things get confusing, because on a vehicle level there are so many things that effect fuel economy, such as transmissions gear ratios and/or final drive ratios, or the transmission driveline overall efficiency. Also bear in mind that a lot of the Brake Specific Fuel Consumption (BSFC) improvements you get under laboratory conditions on an engine dyno won’t necessarily all be seen on a vehicle when the engine is calibrated, as refinement, driveability, emissions and other criteria have to be met.

All other things being equal- which in the real world- they rarely are- using 8:1 compression ratio as a starting point, for every unit raise in Compression ratio- you gain about 3 % in part load fuel economy until about 10:1 when the gains become progressively less. Again this is assuming the cam timing and duration are the same, which they rarely are on a Turbo engine versus a naturally aspirated (cam timing and duration will effect pumping losses and combustion-mostly via EGR).

Power output for power output- again-all other things being equal- when considering a 5.8 litre naturally aspirated engine versus a 3.6 litre twin turbo charged engine (both producing about 400 Bhp)- despite the drop in compression ratio- the smaller engine will have better fuel economy. This is because it is throttled less under part load use- being a smaller engine- and also because under full load conditions this engine is making effective use of otherwise potentially wasted enthalpy out of the exhaust. However it’s not always clear cut, I know a lot down sized Turbo charged engine packages that are running as rich as Lambda 0.6 or 9:1 AFR under full load full speed conditions just to keep exhaust components cool!

 

[Warpspeed]

drwebb, the key words here seem to be "city fuel consumption".

To me that means short trips, lots of cold starts, bumper to bumper traffic, and some short bursts of moderate acceleration away from a fair proportion of traffic lights. I can well believe that the fuel consumption will be very similar for a similar style of driving, all else being equal.

But out on the open freeway at a constant highway speed, the higher compression ratio and reduced exhaust back pressure of the normally aspirated engine should, at least in theory, put you ahead of the game.

 

[drwebb]

OK here are some sales specs that prompted my question:

'02 Volvo S60 2.4 2.4T T5(2.3T)
CR 10.3 9.0 8.5
EPA Mileage 21/28 21/28 21/29

'07 Volvo V70 2.4 2.5T
BoreXstroke 3.27X3.54 3.27X3.67
CR 10 9
EPA Mileage 21X29 20X29

The '07 turbo option shows a mileage debit in the city. I was speculating if the difference in CR was a reasonable explanation for this. However, '02s have the same city rating regardless of engine choice, and the high performance T5 gets a credit on the highway cycle. Judging from Marquis's comment I conclude that differences in mileage due to CR may be present, but marginally small enough to not always show up in the EPA ratings (along with other cofactors).