Here's some basic info on dual-fuel setups using LPG:

Its advocates say that LPG provides more energy than petrol. They are right -- LPG provides roughly eight per cent more energy per unit weight than petrol. But that doesn't mean much, unfortunately. While, theoretically, vehicle operation with LPG should be more efficient than with petrol, this happens only if you've got an engine optimised for LPG. In other words, if you've retro-fitted an LPG conversion unit to your petrol engine, you are not going to be able to get any edge. This is because of the lower density of LPG as compared to petrol and also its higher oxygen demand. The lighter density fuel displaces air in the intake manifold and, consequently, less air per cycle is induced to the cylinders. This results in decreased volumetric efficiency and loss of power, compared to the original petrol rating of the engine.

While we are on the subject, we might as well get one thing straight. Switching to LPG, legally or otherwise, may bring down your running costs. But it will most certainly also drive up your investment cost by trashing your engine. There are, of course, ways of switching from petrol to LPG without distressing your power plant. But I've not come across such technologies in India -- this being the con part of the pros-and-cons equation.

However, this absence of good technology at the workshop level need not bother you if you have a mechanical bent of mind. You could always build an engine -- or, at least, parts of it -- specifically designed to work optimally on LPG. But you need to keep certain parameters in mind.

To begin with, don't even think about a dual fuel set-up. That's the kind of system that is being used in India now -- engines that work on petrol when you run out of LPG. A dual fuel system is pure compromise. As such, it represents the worst of both worlds.

An LPG engine absolutely needs two things: a good ignition system and a good cooling system. LPG requires a very hot spark and the ignition system of your petrol engine simply won't give you one. The engine will also need a good cooling system not because it runs any hotter but because the LPG regulator/ converter uses engine coolant to provide the heat to convert liquid LPG into gas.

We have already talked about the relatively lower efficiency of petrol engines run on LPG earlier in this column. As LPG has fewer BTUs per pound than petrol and as petrol engines are designed to take in a fixed volume of fuel and air, it's pretty much inevitable that you'll be stuck with a ten per cent drop in horsepower. You can eliminate this HP loss by reconfiguring your engine innards -- increased compression ratio, a propane camshaft and so on. But you don't have to. You could, instead, exploit the fact that LPG has an octane equivalent of 110 and advance your engine timing quite a bit.

If, however, you want to go ahead and custom-build an engine designed specifically to run on LPG, do bear in mind that there are three things that are directly affected by fuel choice: compression ratio, cam timing and exhaust valves/seats.

If you're lucky, you should be able to get yourself a higher compression ratio by boring your cylinders oversize and picking up stock pistons from another engine.

As for the camshaft, it serves two separate purposes in most gasoline engines -- it gets as much fuel as possible into the cylinders and it keeps the burning fuel in the cylinders long enough for it to burn completely. There is a short period in the working of a cam during which both the valves remain open -- this is called the overlap. With both the valves open simultaneously, the exiting high-velocity exhaust gas is used to suck in fuel and air. When you are trying to identify the appropriate camshaft for an LPG engine, it might be a good idea to consider trading off high-speed horsepower (or volumetric efficiency) for complete combustion by ensuring zero overlap.

Try and get the hardest valves/seats available as the ignition temperature of LPG is higher than that of pertrol.


As you can see it's best if the engine is setup for LPG rather than compromised to run on both. But without an effective distribution system, as most fleets have, you're constrained to stay near a fueling station or undertake investing in the equipment to store and fill yourself.

I apologize for not posting the ethanol links as promised earlier. Here's a few good links:

Running on Alcohol - Great site and also home of plans for an excellent ethanol still. This is where I got mine.

Journey to Forever - Great site with a variety of bio-fuel info and background on ethanol with some great links as well.

National Ethanol Vehicle Coalition - Industry activists site.

American Coalition for Ethanol - Another industry site, some info.

Renewable Fuels Association - Home of "Ethanol 101" and other industry info.

Governor's Ethanol Coalition - See what state governments are behind energy reform. Lots of good links.