Is cold fuel better ?
Discussion
This is prompted by having driven the Griff in the recent cold weather and being rather surprised at how crisp and responsive the car feels with all that nice cold dense air for it to ingest, seem to recall the same thing with small motorcycles in the dim and distant past. What is the cause of the improved performance with colder air ? For the engine to work well it needs to load a nice clean charge of air at each cycle along with the appropriate amount of fuel. It is in fact probably more interested in a charge of oxygen but that comes along as part of the air. Colder air is denser so a specific volume of cold air contains more oxygen which is fine but the fuelling system doesn’t know that so will presumably within normal operating ranges throw in the same amount of fuel yet it goes noticeably better.
That brings me onto the real question.
Fuel has a fairly high temperature density compared to air and there are those that suggest that because of the large difference in thermal density injecting cooled fuel reduces the charge temperature and therefore increases the amount of oxygen per charge even though there is lot more air than fuel by volume. Were I am going here is.... Is there any point in running a fuel cooler ? I am a bit sceptical because there is about 15 or so times more air than fuel.
That brings me onto the real question.
Fuel has a fairly high temperature density compared to air and there are those that suggest that because of the large difference in thermal density injecting cooled fuel reduces the charge temperature and therefore increases the amount of oxygen per charge even though there is lot more air than fuel by volume. Were I am going here is.... Is there any point in running a fuel cooler ? I am a bit sceptical because there is about 15 or so times more air than fuel.
Diesels often have one, but there isn't much need on petrol, and i think diesels only use them as they inject fuel by the mg, and want to keep the density fairy stable to get the calculations right.
The higher temps will help to atomise the petrol spray, but there was a feature of fuel cooling the intake charge.
I guess petrol fuel temps are pretty stable, so they don't fiddle with them, but diesel temp differences might alter the fueling more.
The higher temps will help to atomise the petrol spray, but there was a feature of fuel cooling the intake charge.
I guess petrol fuel temps are pretty stable, so they don't fiddle with them, but diesel temp differences might alter the fueling more.
David Vizard covered this in one of his mini tuning books. He talked about options for turbo cars such as running a "cold can" which was a fuel line run inside a can full of dry ice in a coiled fashion. It apprently atomised the fuel better or made the induction swirl from inlets to cylinder more stable. This was from a 5 port cross flow A series engine though so the cold fuel relevance is probably changable depending on design.
Cold fuel is heavier though, so you do get a tiny bit more in weight of fuel on a desperatly cold day, as the volume stays then same...
Vizard also gave the formula for 02 denisty per liter over air temp. I believe this was at 0% humidity. As far as i can recall its 5% more O2 denisty every 10 degrees celsius you drop (at sea level).
Knowing about this for a few years i have noticed now i have a decent motor, how much more cleanly and hard the engine pulls on a cold crisp day. Got a decent cold air intake, and i swear blindly you notice a lot more response from the right foot. I think its only noticable on big or highly tuned NA engines. Turbos would just stay a little cooler....
so yes and yes, but its pretty narrow those that will benefit. Your TVR should be loving it, even if the owner is nervous in the snow. (I know i bloody would'nt drive a TVR at the mo!)
Oh and Vizard got about 12bhp from using the "cold can" and icey air on a flow/dyno bench. Thats huge for a mini engine running 150bhp+...
Also I always believe the best thing you could ever do with a car NA or turbo to improve alround performance, is reduce induction air temps.... it just makes plain sense.... and is usally the cheapest option to start....
Cold fuel is heavier though, so you do get a tiny bit more in weight of fuel on a desperatly cold day, as the volume stays then same...
Vizard also gave the formula for 02 denisty per liter over air temp. I believe this was at 0% humidity. As far as i can recall its 5% more O2 denisty every 10 degrees celsius you drop (at sea level).
Knowing about this for a few years i have noticed now i have a decent motor, how much more cleanly and hard the engine pulls on a cold crisp day. Got a decent cold air intake, and i swear blindly you notice a lot more response from the right foot. I think its only noticable on big or highly tuned NA engines. Turbos would just stay a little cooler....
so yes and yes, but its pretty narrow those that will benefit. Your TVR should be loving it, even if the owner is nervous in the snow. (I know i bloody would'nt drive a TVR at the mo!)
Oh and Vizard got about 12bhp from using the "cold can" and icey air on a flow/dyno bench. Thats huge for a mini engine running 150bhp+...
Also I always believe the best thing you could ever do with a car NA or turbo to improve alround performance, is reduce induction air temps.... it just makes plain sense.... and is usally the cheapest option to start....
Edited by GingerWizard on Wednesday 6th January 15:58
Edited by GingerWizard on Wednesday 6th January 16:01
GingerWizard said:
Vizard also gave the formula for 02 denisty per liter over air temp. I believe this was at 0% humidity. As far as i can recall its 5% more O2 denisty every 10 degrees celsius you drop (at sea level)
I'll have to look that up sometime. I wanted to do a number of theoretical calculations regarding the effect of cold air intakes, their efficiency, and the actual increase in power.Rolling road testing is a bit useless for this, as the fan won't supply enough air, and the bonnet is usually open too, too many variables from actual street driving.
Last time i tried to calculate O2 content related to temps and pressures, there was just too much other data, vapour pressures etc, will all move about as the temp and density changes, making it quite an awkward calculation to boil down to just ambient temp vs O2.
I'm a bit surprised his colder fuel help atomisation, i guess it might be due to a bigger thermal gradient and quicker heating of the fuel atomises it better.
I can’t see how it would help atomisation either. I understood the main downside of cooling the fuel was the tendency for it to fall out of the air as it would with a cold engine. A few years ago I found some calculations which used the thermal density of air and fuel and the O2 content change per C and tried to prove the change in O2 per C change in Fuel. Had a look today but can’t seem to find it again. There are some American sites selling dry ice coolers to the drag strip market but they seem a bit confused about where the benefit comes from. Thing is if you get more O2 in then presumably you need more fuel as well which puzzles me. On the road the car feels so much crisper in the cold but looking at the AFR log it doesn’t show as being lean. The map does change the fuelling slightly against air temp but that is mostly when very hot to prevent detonation problems.
I suppose as the O2 per C is known, the test that needs to be done is to throw some cold fuel into an air flow at 15:1 ish and measure the air temperature change. A fairly well regarded engine builder tells me that having air con on a particular car (which also cools the fuel) is good for about 10 Bhp.
I suppose as the O2 per C is known, the test that needs to be done is to throw some cold fuel into an air flow at 15:1 ish and measure the air temperature change. A fairly well regarded engine builder tells me that having air con on a particular car (which also cools the fuel) is good for about 10 Bhp.
350Matt said:
At one point the guys in the pitlane were running big A/C chillers to pump fuel in at -15-20°C there was never anything on the car to cool the fuel directly but as fuel cell temps can reach 50-60 °C so there's quite a gain to be had .
Now all banned of course
Yeah i heard that in the 80's they used to really copol the fuel, but i heard that that was so they could fit more in... maybe that was a load of cr*p lol! Now all banned of course
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