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Old 08-26-2008, 10:59 AM   #7 (permalink)
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Join Date: Jul 2008
Location: NC
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Volvo - '00 Volvo V70 XC AWD SE
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Originally Posted by vtec-e View Post
I had something similar happen my cee'd recently. I blocked the upper grille and the mpg suffered. Its a turbo intercooled diesel so the extra intake heat may have something to do with it. The intake is just behind the grille but at the top of the radiator, drawing air from in front of it. I left the grille block in place and inserted a cardboard radiator block to get the engine hotter. The mpg improved as i reckon the air pressure went up in that area, forcing air into the intake.

By the way, i'm still not getting the mpg i expected. Bleedin diesels! Bleedin warranty too; i cant do bugger all ecomodding to it!

Your instincts are right that bringing in hot air may have hurt your mileage. Diesels are very sensitive to intake air temperature. Hot intake air in a diesel is pure power loss since the limiting factor in diesel engine power is how much O2 there is in the cylinder for combustion. The more O2 the more power at a given fuel rate, and hot air simply has fewer O2 molecules than cold air.

Here is a copy of a post I made on a diesel forum I belong to:

I got to thinking that this was an interesting little experiment so I did it today. I have a van so my under-hood temps will not be directly comparable to yours, but here goes...

Engine inlet temperatures with and without cool air snorkle:

45 mph: snorkle = 84F, engine compartment air = [103F]
65 mph: 84F [101F]
Stop and go less than 30 mph: 88F [136F]

The first number is using the snorkle to pull in cool outside air and the number in brackets is the air temperature of the air pulled from the hot engine compartment with the snorkle disconnected.

The formula used to correct volume given temperature change is called Boyle's Law: (P1*V1)/T1 = (P2*V2)/T2

Using that formula the results from my temperature measurements are:
When the intake air is 19F degrees cooler the engine gets 3.2% more oxygen (nice)
When the intake air is 17F degrees cooler the engine gets 3.0% more oxygen (nice)
When the intake air is 48F degrees cooler the engine gets 8.1% more oxygen (WOW)

For those that do not want to do the arithmetic (and you know who you are ) here are two handy charts I made. To find out how much air (mass, not volume) you are missing out on by not ducting to a cool air source, find the difference in temperature from the hot intake to the cold intake on the vertical axis, follow the line across and then read down to how much less air you are getting from your warm air source.

For example, if your hot intake sends 120F air to the engine and a ducted intake would bring in 80F air, the difference would be 40F. Find 40 on the left axis and move right to the sloping line. Drop an imaginary line straight down and read 0.925. That means you get 7.5% (1-.925 = 0.075 or 7.5%) fewer air molecules from your hot air intake than from your cold air intake.

Here it is in Farenheit, and in Celsius for you Canadians and Norwegians.
So vtec-e, if you increased your diesel's air intake temperature by 19F (~10C) you will have lost about 3.2% of your available power. That will have lowered your mileage since you would have compensated by increased throttle.

What is really interesting is that in stop and go traffic my engine compartment air is 48F hotter than outside air! That would cost me a whopping 8% in power, and a corresponding loss in fuel economy.

Note that this direct application of Boyle's Law v. power only applies to diesels and not to gas engines which have a much more complex response to air inlet temperature.
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Last edited by instarx; 08-26-2008 at 11:06 AM..
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