Heat at Air Intake

[JamesH]

Isn't the value you are looking for called the Specific Heat capacity? It varies according to the substance, not sure how it varies between gases, liquids and solids.

James

[dtype38]

Originally Posted by Chris Y

That reminds me of a vaguely-related question that I've been meaning to ask for some time...

What's easier to change the temperature of - a gas like air, or a liquid like petrol? i.e. for the same volume.

I'm guessing the gas would be, due to lower density? Or am I barking up the wrong tree?

'Fraid that's a bit of a nonsense of a question Chris. Think about it... a cubic meter of air weights 1.2kg, a cubic meter of petrol weights about 740kg. So when you say "easier" are you asking how big a heater you would need to change the temperature? Or how long it would take at a given heat transfer rate? Or how big a heat exchanger you would need to change the temperature at the same at the same rate?

Basically liquids conduct heat much better than gasses so can be heated/cooled more easily, but they're a lot more dense so need a lot more energy to effect the same temperature change.

[Chris Y]

Ooh! you've exposed my fundamental lack of understanding in that particular area But actually your second paragraph pretty much answers my question - thanks

So, effectively, a liquid like petrol, would be quicker to heat up/cool down, but would take more energy to do so. Whereas cooling the air coming into the engine would be potentially slower but require less energy - assuming the same change in temperature for both. Would that be a fair assumption?

[dtype38]

Sort of, but there's a big complication - the fuel:air ratio isn't 1:1. Its more like 14:1 for the engine to run, and that's by mass! So we need around 14kg of air to each 1kg of fuel, which means an air volume of nearly 12 cubic metres to a fuel volume of only 0.00135 cubic metres (1.35 litres).

So for each "revolution of the engine" the energy needed to change the temperature of the fuel is much lower than the air (cos there's a lot less of it) , and its also much easier to do because the fuel conducts heat better.

Regarding the ease/difficulty of transfering heat into liquids and gasses, think about the size of the heat-exchanges on various cars. Liquid/liquid exchangers are tiny (eg, water cooled oil coolers), liquid/gas exchangers are a bit bigger (eg, air cooled oil coolers or water cooled turbo intercoolers), and gas/gas exchangers are huge (eg air cooled turbo intercoolers).

[AU N EGL]

I think cooling the air going into the intake would be easier.

[dtype38]

I thougth that initially too