Manchester XPAG Tests, Modern Petrol – Volatility
The Federation of British Historic Vehicle clubs contributed some financial support to this series of tests in 2016. Paul Ireland has kindly written a summary of his findings for the FBHVC Newsletter reproduced here with kind permission.
To investigate the problems of running classic cars on modern petrol, a series of tests has been run on a 1940s twin SU carburettor XPAG engine at Manchester University School of Mechanical, Aerospace and Civil Engineering.
Petrol consists of over 300 different hydrocarbons. Measuring the volume of fuel that evaporates as a sample of petrol is heated gives a distillation curve for that fuel. The graph below compares the distillation curves of the different fuels used in the Manchester tests and that of 1930s petrol which remained virtually unchanged into the 1970s.
The curve for modern 95 octane forecourt petrol (blue line) compared to 1930s petrol (the orange dotted line) shows that classic petrol is much less volatile, especially at typical engine bay temperatures. This increased volatility of modern petrol is at the heart of the problems suffered by classic car owners.
A petrol engine produces colossal quantities of heat. Unfortunately, only around one third of this heat energy is converted into power to move the car forward, the remaining two thirds is waste heat, most of which goes into heating the engine bay.
At Manchester, the highest petrol temperature in the carburettors when running at full power was 42oC. Not sufficiently high to cause problems.
The thermal image shows the blue float chambers silhouetted against the white (300oC plus) exhaust manifold. Despite being positioned less than one inch above the exhaust manifold, the petrol flowing through the carburettors is keeping them cool. After the engine was stopped, the tests showed the carburettors were being heated by hot gasses coming from the engine through the inlet manifold, not by heat from the exhaust manifold: thus demonstrating that it is not obvious how the carburettors are being heated.
In slow moving traffic, two effects work to increase under-bonnet and petrol temperatures. Although the engine is running at low power and producing less heat, the rate at which heat is lost is reduced, there is less air flow through the engine bay. In addition, petrol is flowing more slowly through the carburettors and has more time to heat up. When the engine is switched off, petrol stops flowing and its temperature will continue to rise as heat soaks out of the engine, exhaust and radiator.
The distillation curve for 95 octane fuel (above) shows a rapid rise in the volume of fuel evaporating between 45oC and 70oC. As the fuel boils, vapour bubbles in the petrol result in the carburettor delivering a much weaker mixture. This is what causes the engine to stop or prevents it from restarting.
The best way to address this problem is to use a petrol with fewer low temperature components, more like the 1930s petrol. This reduces the volume of fuel that will boil as the engine bay gets hotter.
Unfortunately, it is not possible make any specific recommendations for two reasons. Firstly, the regional UK fuel distribution industry is served by around 14 different refineries, all of which produce slightly different base stock. Secondly, there are three different grades of fuel are sold throughout the year:
- Winter fuel – October to April.
- Intermediate fuel – April to May and September to October.
- Summer fuel – June to August. This will probably have fewer low temperature components.
In practice these dates are not fixed and will vary with ambient temperature and the turnover at any particular filling station, making it virtually impossible to know what grade of petrol is being sold.
The data above indicates super grade fuels are possibly less volatile. However, it is worth trying different brands to find out which petrol and grade gives the smoothest performance and will reduce vaporisation problems.
It is important the engine is properly tuned. Even a few percent reduction in efficiency, probably not noticeable in normal road use, will increase the amount of waste heat. The cooling system should also be working efficiently. Electric radiator fans help keep air circulating but may make matters worse. In slow moving traffic, they are drawing hot air through the radiator and blowing it into the engine bay. It is also worth fitting a timer or equivalent circuit to ensure any electric fans continue to run for around 5-10 minutes after the engine has stopped.
Anything that can be done to keep the fuel system, particularly the carburettors, cool will help reduce the severity of the problems caused by the low temperature volatility of petrol. An infrared thermometer or thermal imaging camera is the ideal way to identify hot spots. Unfortunately, as soon the bonnet is opened, the temperature profile will change. As an alternative, digital multi-meters with thermocouples are now inexpensive and provide the means to allow your passenger to accurately measure the temperature of the fuel system even while a car is moving.
Unfortunately, there is no magic solution to this problem but with care it is possible to reduce its severity.