Petrol in Diesel Engine

[Mark Evans]

Which is how such engines are used in turbo-prop aircraft and helicopters.

[Mark Evans]

It means that the output of the engine changes after the throttle has been changed. Which isn't the kind of behaviour people expect from a car engine. (Possibly even those familiar with piloting jet aircraft would have problems driving a car which behaved in this way.)

[Christian McArdle]

The latter. It can take up to 20 seconds for a turbofan to reach full power. This would not be good for overtaking. However, this is somewhat misleading.

The lag can be countered for by careful design of the transmission. This is because it is the acceleration of the components that takes time, not the variation in input power, which can be changed rapidly by varying fuel flow.

Power can be varied much more rapidly if the engine is allowed to rotate at constant speed. This enables their use in helicopters and provides much better control in turboprops, where power can be almost instananeously altered with the use of variable pitch (constant speed) propellors.

In a land vehicle application, the use of a variable speed transmission would have a similar effect. Instant acceleration would consist of rapidly increasing the fuel flow and adjusting the transmission to keep the engine at the constant speed (coordinated by a FADEC). Response to the throttle would be determined by the transmission's ability to immediately adjust ratio, not the response of the engine.

Obviously, total acceleration is limited by the total fuel flow that can be pumped into the engine. If this is exceeded (but ratio adjusted regardless), the engine will slow down. This could be beneficial, though. There would be some ability to get extra "overtaking" boost by accepting a certain LP shaft reduction. This might allow a smaller engine for the same application.

I think the real problems with turbine land vehicles would be cost, gyroscopic effects and safety considerations from the high energy contained in the rotating engine structure (uncontained engine failures, crash worthiness etc.).

Christian.

[Peter Ashby]

Excuse my ignorance, but how does all this relate to Mazda's rotary car engine?

Peter

[elgorrion]

I did it yesterday - put about 4 gallons of unleaded into my LDV20 (sherpa) and topped up with about 3 gallons of diesel. There wa probably 2-3 gallons already in. OK for a few miles then very slow an smoky in 1st and 2nd, better in 3rd and 4th - not chanced 5th! Been trying to find out if adding engine oil, or some form of pur mineral oil (or olive oil a la biodiesel) would help lube the pumps an make the mix more diesel-like, and if so in what ratio to the petrol? Worried

-- elgorrion

[Derek ^]

2 Possibilities.

1) The performance deteriorated as the diesel fuel in the lines and filters got replaced by petrol/diesel mixture which did not burn as predicted in the design of the engine causing pre-ignition, detonation, or whatever.

2) The performance deteriorated because the petrol in the mix had damaged the pump/injectors.

:(

FWIU in summer, the cheapest veg oil is more or less equivalent to diesel, at least as far as half a tank goes. Proprietory motor oil includes additives that might not agree with seals in the fuel system.

If there's room in the tank why not add more diesel, (Why start experimenting with veg oil at a time like this?).

Better still, try and make room in the tank by removing all but half a gallon or so of contaminated fuel, (To avoid getting air in the fuel system) then filling right up with diesel. If you are parsimonious the fuel so removed could be used over time at a rate of say a gallon/tankful.

Intuitively I'd expect it to cause less damage if it was left ticking over whilst it purged the contaminated fuel from the lines/filters etc. rather than thrashed down the Motorway. ;-)

Let us know how you get on.

DG

[dennis]

It was (still is?) common practice to put upto 30% petrol in diesel cars to stop waxing in very cold weather. Isn't this what winter diesel is anyway?

[Mark]

Did you know that contaminated fuel like this is Free All you need do is replace your engine with a Rolls-Royce K60 :-)

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[Steve Walker]

10% in a rugged old Sherpa? It'll be fine.

[John Rumm]

4 gallons out of 10 in total sounds more like 40% to me ;-)

[The Natural Philosopher]

I'd say so as well Just keep topping up with diesel till she runs adequately, then run her to nearly empty.

[John Schmitt]

It was normally paraffin used for wax-proofing diesel and the Excisemen could be very cross with you for this. Another genius method was to light a fire under the fuel tank of a truck. Winter diesel is normally a lower-boiling cut of the distillate which does not particularly carry the alkanes present in petrol, more paraffin sized ones. In fact chromatograms of distillate fuels show a characteristic bell-curve shape across the peaks as in

Winter diesel I worked with carried a double peak, suggesting strongly that it was a blend of two distillates, one being in the standard diesel range of carbon numbers, and the other being slightly on the heavy side of paraffin.

John Schmitt

[Badger]

I'd add a gallon of cheap 20-50 to rebalance the mix meself...