Fuel Cooling Ability

The number in the column titled “Latent Heat of Evaporation” indicates the fuel’s ability to absorb heat from both the air and from the engine as it vapourises. The higher the number the more effective it is.

A quick glance down the column seems to show that Methanol is the best at internally cooling an engine. But, as with Fuel Ratings, there is more to it than meets the eye. We have to take into account the Air/Fuel ratio to get the full picture.

To have a true indication we must divide the Btu/lb figure by the A/F ratio. For instance, with Methanol having a Latent Heat of Evaporation of 472 Btu/lb and an A/F ratio of 4.5 this gives a figure of 104.9 (the number in brackets in the Fuel Characteristics chart).

Using the figures for petrol we get 10.8 which shows that Methanol (104.9) is almost 10 times more efficient at removing heat. For this reason a petrol fuelled engine is much more reliant on cylinder and head fins to radiate heat, a point of concern when converting a glow engine to spark ignition.

Now consider the figures for Nitro Methane. Even at the leanest A/F ratio it is almost as good as Methanol (103.2) but at its richest setting it has 5 times the cooling effect of Methanol (516). This helps explain why an engine using nitro can not only develop more power but may also run a little cooler.

Brian Hampton

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