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Cooking up a smooth drive

Pratap Ravindran

My office happens to be located on a main thoroughfare lined with big-name retail outlets and I, therefore, get to see a lot of rich people out shopping. They come in big, expensive cars with snooty chauffers at the wheel. Some of the cars are imported, many are not. But virtually all of them have one feature in common: they run on diesel.

Odd, isn't it? I mean, here are people who own several cars each of which costs a few lakh rupees. One would imagine that they would go in for petrol cars which tend to be smoother. Cost, after all, should not be a consideration for them.

That is the amusing part. Now for the intriguing bit. As in most other areas of human interaction, there seems to be a weird hierarchy at work here. The diesel car types look down on the petrol car-wallahs. And the LPG car people sneer at both! Nobody, i ncluding the LPG car types themselves, is very clear about why this hierarchy is structured the way it is. After all, there is nothing particularly chic about driving around with a cooking gas cylinder concealed in your boot.

And then again, I didn't think it's any ``politically correct'' consideration at work. For the most part, I find that the kind of people who run around in LPG-powered vehicles are the kind who crib like hell about the poor maintenance of Government-owned mass transport vehicles. That's where their concern for the environment begins -- and that's precisely where it ends.

The mystery persists

Meanwhile, my mechanic tells me that he is doing a roaring business in LPG conversion. Mind you, it's illegal to use LPG to power your car. But that doesn't seem to stop people from switching to it.

As an Arab potentate is said to have remarked on being introduced to the game of cricket, ``God is great!''

Whatever be the reason(s) for people switching to LPG, the fact remains that any move away from petrol is a good thing per se. However, I'm not very certain that a switch to LPG is such a good thing. More precisely, I am not convinced that the technology that we are using in this country to switch to LPG is a good thing. You will understand why as we go along.

Good thing or not, I understand that the Central Government is understood to be moving to amend the law so that LPG-powered vehicles become legal. So we might as well figure out what a conversion to LPG means.

What follows is basically a primer on LPG as an alternate fuel for vehicles. Such a primer is necessary because I find that not many people -- including those who have already switched -- are aware of the pros and cons.

Let us start at the beginning. LPG or liquefied petroleum gas is often identified as propane. It is not. This confusion has come about because LPG in the US and Canada consists primarily of propane. In India, LPG is a mixture of petroleum and natural gas es that exist in a liquid state at ambient temperatures when under moderate pressure (say 200 psi).

Natural gas processing and petroleum refining constitute the major sources of LPG. Raw natural gas often contains excessively high levels of propane and butanes which have to be removed to prevent their condensation in high-pressure pipelines. In petrole um refining, LPG is collected during distillation from lighter compounds dissolved in the crude. It is also generated in the ``cracking'' of heavy hydrocarbons. LPG, therefore, can be described as a by-product whose exact composition varies with the sour ce.

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 lo wer 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 a nd 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 kee p 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 wo rlds.

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 becaus e 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 i nevitable 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 th e 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 work ing 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 I'm dead set against dual fuel set-ups, I hope the Government, as and when it gets around to legalising LPG as a vehicular fuel, insists on a single fuel system. If this happens, you will have to fit an LPG fuel tank -- as against the current practice of lashing down a cooking gas cylinder in your boot with velcro straps. Whatever be the means by which you install an LPG fuel tank, take care to ensure that it is outside your car. Hoses must also run outside the passenger compartment of your vehicle. If you insist on trunk-mounting the tank, seal off the passenger area with foam. And have vents cut to allow LPG leakages to escape.

Before we wind up, let's look at a critical question: How clean are LPG engines?

LPG was first introduced in markets abroad as a clean-burning fuel. That was quite some years ago. At that time, LPG was, in fact, cleaner than the other fuels available. For one thing, LPG resulted in a lower emission of carbon dioxide as compared to pe trol -- but not to diesel. And then again, it eliminated heavy hydrocarbon emission and lowered the generation of toxic air contaminants such as benzene and 1.3 butadiene. Finally, it offered zero evaporative and running losses as it worked in a sealed f uel system.

In recent times, however, alternatives such as natural gas (and, of course, diesel and bio-diesel) have gained acceptance, as a result of which work on the development of dedicated LPG engine technology has slowed down.

In any case, I'm personally betting on hybrids. So don't rush out and buy yourself another LPG cylinder just yet.

Pic.: The common practice is to strap an LPG cyclinder in the car boot.