INTERESTING QUESTION...
 

I thought of this the other day, and would like some (educated) input on it as I would like to know the answer.

IN PRINCIPLE a piston engine works the same way as a car engine - right?
So on a cold morning when you start your car, why does your car's exhaust spit out steam, but the same does not apply to a piston aircraft / Helicopter?

69

I'd imagine because the propeller blast blows it away. Oh, hang on - what about Robinsons, before the rotor clutch is engaged...

Maybe it's to do with the short length (and angle) of the exhaust pipe not allowing condensation to build up. I'll have to think about this a bit more.

I suspect that it's a temperature issue.

The presence of condensed water vapour in the exhaust indicates that the water produced by combustion has been cooled below it's saturation temperature, which is a function of absolute humidity.

A car has water cooled cylinders, which take longer to warm up than air-cooled and there is a longer length of exhaust system, allowing more heat loss before the gases get dispersed in the atmosphere.

Aero-engines are more comparable to motor-cycles in that respect.

That makes sense, thank you very much!

Sticking with the car/airplane similarity theory;

How come carburettor operated cars don't have a problem with carb icing? Or do they? Or is 'carb heat' applied automatically all the time?

Question that confuses me is:

Dewpoint is when it rains when temp gets lowered to dew point.

but what makes the dew point change, if it's pressure then i thought pressure is temperature.

like it rains when it's boiling hot aswell.

Carburettor cars do suffer from carb icing. I used to have a zooped up mini (a proper one, not a small Bavarian) and on cold winter nights it was not unknown for it to suffer icing. In the end, I had to fit a water heated inlet manifold to cure the problem!

As for steam etc from car exhausts. This is simply water vapour that has collected in the exhaust system, especially the silencer, being expelled.

Simplified explanation... After the car is switched off, the exhaust is still hot/warm. As it cools, the vapour in the air condenses on to the surface of the metal. Then when you start the car, it is forced out as the "steam" you see on cold mornings.

Regards,

Shuttlebus

Carbureted cars built since the 70s have an alternate air inlet controlled by a vacuum motor which in turn is controlled by a thermostatic switch or valve placed in the air stream. The alternate air is drawn across a stove built onto the exhaust manifold. Older models had a bimetal spring controlling the alternate air valve.

Put succinctly - Automatic Carb Heat.

Exactly ! - also aircraft engines tend to have a hotter exhaust than their automotive counterparts. Silencer contains numerous passages/baffles to collect moisture after shut down - errm - turning the car off and it is this moisture that turns to steam once the engine is restarted. This is why a standard (non-stainless) exhaust system on a car driven on the highway will outlast its city driven counterpart.

Must've been loads of fun ! :)

Had carb icing on a Triumph TR7, going down the motorway doing about 70ish mph, the throttle jammed open. Fortunately not far from the services so managed to give it a load of brake and dab clutch until clear of motorway. As i coasted to a stop it cleared. It was an early version of the car, did not have the hot/cold valve on air box, so routed the inlet trunking to the exhaust manifold.
Also have a Kawasaki gpz900, when bought it was 15 years old, but Kawasaki still honoured a call back to replace the carbs. with ones with built in hot water circulation to prevent carb icing. Apparently there had been cases or carb. icing, reducng power and locking up the back wheel.

Also read somewhere for every gallon of fuel burnt 8 gallons of water produced? I'm assuming from condensation from the air drawn through the engine.

Car silencer systems tend to accumulate condensation within them - that's why they rust so well.

Most aircraft engines don't have silencers.

G

I tend to disagree, I think that it is the relatively cold exhaust pipe that lowers the water vapour in the exhaust to below it's dewpoint. As the pipe heats up, so the condensation reduces. I don't think that there is enough water in the exhaust pipe to generate the amount of condensation that we see on cold mornings as there is insufficient volume in the exhaust pipe. But going back to original question - what about the white trails we leave behind in the sky? Yes, water is produced as part as a by-product of burning hydro-carbons.

Piltdown man,

You are exactly right, as part of the burning process, hydrocarbons and water are a by product. As a result on a cold day the water vapour can condense and result in water dripping from the exhaust. Now in theory a well maintained engine shouldn't produce to much water vapour as all the Oxygen should be burnt off, however if the mixture is not correct then you may get a situation where more water can be produced due to excess oxygen.

Again car exhaust are longer and therefore allow the condensation to occur in the exhaust rather than an airo engine which has a short exhaust and thus blows it into the atmosphere.

Check your chemistry...

Hydrocarbons will produce a fixed amount of water when combusted. It's purely a function of how much hydrogen is in the fuel. If there is enough oxygen available, it will pretty much ALL form water vapour.

When I were a lad, Esso had some publicity material whcih said that you got 8 gallons of water from every gallon of petrol. As a lapsed chemist I should be able to work it out but it sounds about right.

118g of petrol produces 180g of water, so nearer 1.5 gal water to 1gal of fuel

C8H18 + 12.5 02 --> 9H2O + 8CO2

Carb icing was common on many carby motorcycle engines through the end of the last century. My GSX-R-750 WV would do it in the right conditions, for example. Nowadays everything's fuel injected (sadly!), so this isn't an issue.

Hi there
Working on aircraft engines, I would say first of all aircraft exhaust pipes are very short just over a meter or so long from the last cylinder compared to cars and as a result any heat is dissapated very quicky after engine shut down, so there is no place for condensation to really collect as you have in a car system with silencer boxes. If there was any vapour it would be hard to see as with aircraft engines(horizontal apposed engines) tend to collect oil in the barrels and on start up you usually see alot of white smoke from the exhaust as the engine starts and the exhaust pipe gets hot very quickly, if you look on the ground you will mostly see traces of oil after the aircraft has moved away. Car exhausts take much longer to heat up and as a result water collected is slowly explelled as the pipes heat up.



Hope that helps

A330 asked " Question that confuses me is: Dewpoint is when it rains when temp gets lowered to dew point. but what makes the dew point change, if it's pressure then i thought pressure is temperature. like it rains when it's boiling hot"

Nobody else answered. I am not an expert, but I think:

(a) Dew point is not when it rains. Dew point is when tiny droplets form, as air with some moisture in it gets cooled down. The tiny droplets may stay like that as fog, coalesce on solids such a grass blades and form dew, or (at height) coalesce as raindrops or hail.

(b) Dew point changes with humidity. "Dry" air has little moisture and a very low dew point. Dry air which passes over water, e.g. coming from the USA across the Atlantic, picks up water vapour on the way and reaches higher humidity.

Water in moist air stays in the form of water vapour unless the temperature drops, in which case the relative humidity rises. When RH reaches 100 percent, i.e. air temp = dew point temp, droplets form. One example is air reaching Welsh hills, gets pushed up, and forms clouds. If conditions are right, droplets coalesce as raindrops and it makes Welsh rain. The air, having lost some moisture, gets lower (absolute) humidity. RH is still 100 percent while the air is as high as it got. When it descends on the downwind side of the hills, temperature rises, water content/humidity stays at the post-rain level, but dew point is now lower - so England gets less rain than Wales in prevailing west winds. Over England, the medium RH air gets cooled in thermals as it rises, typically forming cumulus clouds, but is much less likely to rain unless, e.g., cu gets very tall - hence cu nim = rain + hail.

Chris N.

thank you everyone!