Heated fuel filter

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After several winters of draining and swabbing HMPEs or wax out of the fuel tank in sub-zero temperatures, the writer decided to make a heated filter to combat the problem.

The vehicle is a 1998 Land Rover Discovery, which takes for ever to reach working temperature if the engine isn’t under load. Unlike running on vegetable oil the car will start fine on cold bio, the problems come when precipitated wax/HMPEs are drawn from the tank and block the filter, so both electrical and water heating was used with the thinking that once the engine has started the filter needed to be warmed as quickly as possible and with ample electrical power available, an electric heater should fit the bill until normal running temperature was reached.

Hunting round for cheap 12v heaters yielded one of those mini immersion heaters which plug into the cigarette lighter and dunk in a cup of water to make hot drinks.

A 200 watt item was ordered from a well known internet auction site for about 4 or 5 GBP and tested. Disappointingly it took about 10 minutes to warm a cup of water to around 60°C, alternative heaters were sought. After a little investigation Peltier cells seemed like a good alternative.

4, 60 watt, 40mm square cells were ordered from a Chinese supplier via the auction site at around 4 GBP each. A quick play with these when they arrived was fascinating. Within 30 seconds the hot face is too hot to hold and with 90 seconds it was up to 130°C. So the decision was made to fit both the "Cup-a-soup" heater and three Peltier cells.

On the Discovery, the fuel flow through the filter is in at the top, down through the paper element to the bottom and it returns to a top outlet via a pipe through the centre of the element. It’s not known if that’s the same for all cars, possibly not.

In operation its electrical performance is better than hope for. It rises the temperature from 11°C to 35°C in around a minute, but slows after hitting 36-37°C. It is assumed this is a function of the Peltier’s behaviour. This temperature rise seems very good considering that at start up cold engine coolant flowing through coil and cold fuel is being introduced into the chamber.

Performance with water heating, on the other hand is rather disappointing. As expected, warm up is slow, but once the engine is up to temperature it seems to fluctuate with in the 30°C’s range but will hit the low 40°C’s on a motorway.

As far as can be ascertained, most HMPEs melt at or below 30°C, so it was hoped the heater might run at around 40 – 50°C to ensure they are melted. It would seem the poor performance has much to do with the coolant flow through the coil, because when hot and the engine switched off the temperature can hit 60°C, presumably by thermo siphoning.

Julian 22:12, 3 November 2012 (GMT)