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Classic Engines, Modern Fuel

Comments on Topic: Petrol volatility and under bonnet temperature.

Submitted by David
25-Jun-2020

The TC has about 10 inches of copper fuel feed pipe to the petrol pump under the bonnet whereas my TA has 46 inches and the pump is situated on the hotter side of the bulkhead.

This prompted me to measure the fuel tempurature arriving at the carbureters. I introduced a sensor [ 3" from the petrol pump top exit ] into the pump to carburetor pipe. I also measure the rear float chamber temperature, which represents the general under bonnet temp.

My first test run was in 22 deg C ambiant, a warm day. My second was in 30 deg C ambiant air, a hot and sunny day.

The first run showed that the temperatures would not have caused any trouble but with the hot day, slow traffic and short stops, the temperatures of the fuel rise to uncomfortable levels. Although my TA restarted ok I can imagine some cars would find dificulties. Page 18 of Pauls book shows a petrol volatility graph.

Results. Second run, Ambiant temp 30 [ temps in deg C ] Fuel BP 97 probably summer mix !!

Home to layby 12 mls, 40 mph, radiator 70 deg. -- fuel temp 44 float chamber 41.

Layby stop 3 mins -- fuel temp 48 float chamber 39. restart ok.

Run to next layby 8 mls, 40 mph -- fuel temp 44 float chamber 46.

Layby stop 10 mins, rad 75 -- fuel 60 float chamber 51. restart ok !

Run home 8 mls 30mph [ slow traffic ] --fuel 50 float chamber 48.

I have a home made exhaust / carb heat shield to keep the float chambers, paticulrly the rear, cool.

David Heath.

Submitted by Anonymous
25-Jun-2020

Interesting. The fuel is often hotter than the bowl. But not always.

I thought that the fuel was acting as a coolant for the bowl. It is not the case. Where is it heated ? In the pump ?

Recently, I installed insulating sheath all along the fuel line (along the exhaust, before and after the pump). leaving the pump as is. No measurement, but I think it had no effect for the bowl temperature.

Are you sure the temperature you get from the sensor of the pump is the temperature of the fuel and not the one from the copper pipe ?

Paul, do you think there is a gradient between the pipe and the fuel inside ?

Laurent.

Submitted by The Author
26-Jun-2020

David has posted a very interesting set of data. To answer your question, we need to remind ourselves about how heat is transferred from a cold body to a hot one. There are three mechanisms.

  1. Radiation - (e.g. the heat from the sun). This is the most effective of transferring heat but it is very dependent on the temperature of the radiating body. Less than say 200C and there will not be a great deal of radiated heat.
  2. Convection – (e.g. water boiling in a kettle). This is also a very effective way of transferring heat, but it only occurs in fluids. A hot fluid is less dense than a cold one. If you heat the bottom of a container, the hot fluid rises transferring the heat to the upper cold fluid. This is how heat is distributed in the float chamber where there is a significant volume of petrol.
  3. Conduction – This is the slowest way of moving heat. Metals are the best heat conductors. As there is only a small volume of petrol in the fuel pipes this is probably heated by conduction. Petrol is a poor heat conductor. It is probably there is a heat gradient between the petrol on the outside of the pipe and its core.

However, I do not believe this is the reason for the differences in temperatures measured by David. In the fuel hose from the pump to the carburettor, the fuel is under pressure, which raises its boiling point. As the pressure of the 60C petrol drops when it “sprays” into the float chamber through the needle valve, it is possible up to 20% will evaporate, cooling the float bowl and petrol. Hence the temperature of the float bowl and petrol is lower than that leaving the pump.

Submitted by The Author
26-Jun-2020

Yesterday temperatures in the UK were 33C. I was out in my TC and got held up for about 30 minutes in a very slow moving traffic jam. As might be expected the radiator temperature went up to 90C. The electric fuel pump started clicking furiously as it tried to pump against vapour in the hoses between the pump and carburettors.

However, the engine continued to run OK. Something I was not expecting. The possible reason for this is interesting.

Only when petrol boils in the jet will it cause running problems. Should the petrol boil in the petrol pump, hoses to the carburettors or float bowl, the vapour will escape through the carburettor breather pipe and not enter the jet. Only the higher boiling point fractions will remain and pass into the jet, reducing the risk of it also boiling.

Suggesting that insulating the petrol hoses or moving them to cooler areas of the engine may make the Hot Restart problem worse. Doing this will allow the lower temperature fractions to enter the float chamber and possibly the jet.

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