Diesel - Not just for trucks

Merc S-Class

Okay, alternative fuel might be a misleading category in which to place an article about diesel, however in the context of the Thai passenger car market, diesel is very much an underutilized alternative to the petrol engine.

In Europe there are very few cars on the market that don't have at least one diesel-engine option and in many cases the diesel-engined versions are the top sellers.

In Thailand there are only a handful of passenger cars that offer diesel power, and none of them are mainstream models. Why is it taking so long for diesel to catch on in passenger car transport? Let's take a look.

What's so great about diesel engines?

Diesel offers several advantages over petrol, the main one being far better fuel economy. How?

The first reason that diesel offers better economy is because the fuel has a higher energy density than gasoline. On average 1 liter of diesel contains about 38,000 BTU, compared to a liter of petrol containing around 33,000 BTU of energy.

What's Common Rail?

Common Rail Diesel
One of the main factors in determining the efficiency of a diesel engine is the way the fuel is injected into the cylinders. The pressure the fuel enters the cylinders and the configuration of the injectors are the two key factors.

A common rail injection treats these two issues separately. First the fuel is stored under high pressure in an accumulator rail, which is then used to supply fuel to the cylinders. Electronically controlled injectors are used to deliver the fuel from the common rail into the individual cylinders. This system allows high injection pressures to be available at all times, increasing the efficiency of the engine.

Secondly, diesel engines are capable of operating at much higher compression ratios than petrol engines. Actually it's about double, with petrol engine compression ratios of around 9:1 being common, and diesel ratios averaging around 18:1.

Basically, the higher the compression ratio, the more power will be generated (from the same amount of fuel).

In a petrol engine a fuel/air mixture is added to the chamber and compressed together. Compressing the air too much will case the fuel to ignite, which would cause engine knock. In a diesel engine only the air is compressed before the fuel is added.

The combination of these factors gives diesel engines an unbridgeable economy advantage over petrol engines.

Additionally, diesel fuel is generally cheaper than petrol since it requires less refining.

So what's keeping it back?

In Europe, diesel passenger cars have been around for a long time, but historically they have not had a big following. Diesel knock, lack of refinement and inconvenience made it an unpopular choice for passenger cars. Modern diesel engines have overcome the major problems, offering refinement levels virtually on par with their petrol counterparts. Diesel engines once required a long heating period of around 30 seconds before they would start, but again this issue has been overcome and has long been a thing of the past.

Recent advances in diesel technology including common rail injection and multi jet technology have also vastly improved the efficiency of diesel engines, and have helped diesel engines to burn the fuel more completely, resulting in fewer emissions, and of course, greater efficiency.

Maybe it's about perception

Audi R10

A year ago there was a sign for Toyota Yaris bookings at the local Central. Toyota had a display of their Vigos, Fortuners, and of course a few Altis, Vios and Camry models as well, and a sign telling us that the Yaris was on the way and we could book one now. At the time I had read somewhere that a diesel Yaris might make it to Thailand, and I was aware that the Yaris was already on the market in Europe with a very efficient 1.4 liter D-4D engine. I proceeded to ask the sales lady if there was to be a diesel engine option for the Yaris. "No, no... it's petrol....... erm.... it's a passenger car, not a pickup!" She looked at me like I was silly for asking.

It's almost as though the Thai psyche equates diesel to pickup trucks and petrol to passenger cars. Could it be that simple? The market simply doesn't understand the technology, and doesn't want it.

But diesel engined passenger cars are available in Thailand.

High end first?

If perception is the problem then a healthy beginning in fighting it off has started. The top proponents of diesel engines for passenger cars are Mercedes-Benz, Volvo and BMW.

Mercedes recently chalked up 100 orders of their locally-assembled diesel powered S-class during their "Mercedes-Benz StarFest 2006", from the 9-12 November. Not at all bad for 4 days considering that the S-class is Mercedes flagship, and is aimed at the kind of folks who don't really have to worry too much about money. Was the 15km/l fuel economy really a key selling point on a car that costs THB7.29 million? I suggest not.

The S320 CDI is powered by a 3.0-liter common rail direct injection engine, producing 211hp and 540Nm of torque. This is enough to propel the car from 0-100km in a brisk 7.5 seconds.

Any negative perception issues surrounding the suitability of diesel engines for use in passenger cars should be shaken off once the rich and famous start cruising around in their diesel powered luxury cars.

So what about us?

The rich can get their hands on them, why can't we? Given that diesel's main selling point is efficiency and economy, why not start offering them to people who would actually benefit.

Now is the time for diesel to make a real entrance on the Thai passenger market. The big manufacturers all have diesel engines in their arsenals, and it would seem that there is no good reason why they can't simply start offering them in some of their models now.

Perhaps the only thing holding them back seems to be a belief that nobody wants them. Please, automotive market research people, ask us again. You might find out that we actually want diesel cars.