Air density and power

[steve-V8s]

Colder denser air makes more power so you fit intercoolers, keep the inlet manifold cool and suck air from outside rather than under the bonnet. I understand that a cooler charge reduces the chance of detonation so the ignition can be advanced but does Cool Dense air itself actually make more power ?

Is additional power only possible if the ECU adjusts the ignition advance after looking at the inlet temps and having a listen to the knock sensor (if you have one)

I recall from the dim and distant past being convinced that small revvy bike engines running on carbs with no electronic gizmos felt nice and crisp when the weather was cool and particularly when it was a bit misty.

[PeterBurgess]

For every 3 degrees cooler you can make 1% more bhp as the air is more dense....tighter packed molecules....more O2.

To get best power the fuel needs to be spot on.

Here is a link http://www.dtec.net.au/Downloads.htm download the Air density and tuning, jetting with RAD

Peter

[SuperchargedVR6]

steve-V8s said:

Is additional power only possible if the ECU adjusts the ignition advance after looking at the inlet temps and having a listen to the knock sensor (if you have one)

I don't think ECUs actually add advance. I thought they were just mapped for Mean Best Torque and then just retard the base map when things get hot and sweaty?

I know you can do that on standalones, but I'm sure OEMs play things safe?

[stevesingo]

MBT will be different for 5degC than 35degC, likewise for 990Mbar and 1010Mbar. OEM calibration is conducted in enviromentally controlled dyno cells where they can calibrate compensation table accuratly for IAT and Baro.

The best modern stuff (E90M3) has ion sensing technology in the ecu, effectivly measuring incylinder pressure via the spark plugs, thus constantly changing ign advance. There must be some dyno time involded that is for sure.

IIRC, I read in Autocar that Bently states that £1,000,000 could be spent on on fuel alone in developing an engine. The dyno time would be without foad fuel tax presumably!

[steve-V8s]

Ok let me ask the question slightly differently.

If I have an engine with fixed fuelling and fixed ignition advance which is optimum for the current conditions and I reduce the induction air temperature will the power increase? I would guess it would reduce because there is now more oxygen but the same amount of fuel so the A/F ratio is no longer ideal.

What I am thinking about here is a fuel cooler. The engine I am working with does not have a knock sensor, the only information available to the ECU to protect against detonation is induction air temp. If I squirt in cooler fuel in an attempt to cool the air for a short period of peak power (about 30 seconds) is there any benefit as the ECU will not know it has happened.

I can only see a benefit if the fuel is always cooled and the engine mapped accordingly. Suppose if I could quantify any change and adjust the inlet temp reading that would do the job, it is rather difficult to measure the air temp in the manifold after the injector.

[PeterBurgess]

In the real world where there are many shades of grey especially in terms of afr it will most likely make more power if it gets cooler air. In the theoretical world it might well lose power!

Peter

[Pumaracing]

steve-V8s said:

but does Cool Dense air itself actually make more power ?

Assuming correct fueling, engine power varies fairly directly with the mass of air inducted per cycle. So cool dense air does indeed make more power because it leads to a greater mass of air (and therefore fuel) in the cylinder available to be burnt.

Over the usual range of temperatures we encounter over here air density rises by 1% for roughly every 3 degrees C fall in temperature. However don't fall into the trap of thinking that means engine power also rises by 1%.

Because the air is more dense it also flows more slowly at a given pressure drop across the engine so the density rises by 1% but the volume inducted per cycle falls by about half that leading to only about a 0.5% increase in mass inducted. In fact it's a square root law that comes into play which means it actually takes approx 10F or 5.6C fall in ambient air temperature for the engine to gain 1% in power.

The actual dyno correction factor if you measure bhp at temperature T1 and want to correct back to what bhp would have been at temperature T2 is (in degrees C):

sq rt ((273.15 + T1) / (273.15 + T2))

[stevieturbo]

SuperchargedVR6 said:

I don't think ECUs actually add advance. I thought they were just mapped for Mean Best Torque and then just retard the base map when things get hot and sweaty?

I know you can do that on standalones, but I'm sure OEMs play things safe?

Some modern ecu's can add and retard, they dont make things simple these days ! lol