Mario James

SINCE THE onset of Desert Storm - The Sequel, the price of dino oil has been pushed slowly beyond the outstretched hands of most salaried folk. Like an avalanche, the price increase on this basic commodity has had a ripple effect globally, pushing inflation to unheard of heights.

Economising is a renovated and repolished buzzword that has filtered down to what, and how, we drive. In some countries, owning a large SUV is considered antisocial behaviour. In others, diesel technology has become the weapon of choice in the consumer's war against spiralling fuel costs.

Diesel engines operate on a process that is modified from the Otto cycle. Simply put, the more you can compress a fluid, the more it heats up, and the more efficient the process becomes.

These engines run on a compression ratio (ratio of volume of cylinder to that of combustion chamber) greater than 15:1, possibly even greater than 20:1. At this level, the air gets so hot that when fuel is injected, it instantly combusts. The process is more efficient theoretically, because more heat seems to be converted from the burn to do useful work, i.e., move pistons. Some manufacturers claim a more than 40 per cent improvement in fuel consumption over comparative gasolene engines.

diesel engines

Most of Europe seem to have bought into this thinking, as cars on the continent equipped with diesel engines have been outselling their gasolene motor counterparts since 2006, when cars equipped to burn the heavier oil garnered 50.6 per cent of the market, according to the BNET business network. But, is diesel the godsend that purveyors of the technology would have you believe?

To attain this higher compression ratio, diesels have to be built stronger to withstand the greater combustion pressures and resultant forces on the crankshaft, connecting rods and bearings.

Pound for pound, they are heavier units because of the increased mass in these areas. While this does wonders for engine durability, it means that compression ignition engines turn slower than their pure Otto cycle counterparts. And because of the huge combustion force, they tend to be noisier mechanically, as the piston tends to slap the cylinder walls on the power stroke, resulting in the well-known 'diesel clatter'.

oil burners

What it comes down to is this: The oil burners make more torque because they squeeze the intake charge more, and they develop more torque at a lower rpm than gasolene engines do.

They are most efficient at what would be considered idle speeds on gas engines, and cars that they reside in are geared to take advantage of this. They also have no butterfly valve (throttle); engine speed is governed by controlling how much fuel is injected into the combustion chamber, and this eliminates air flow losses at this lower rpm.

Since the cylinder is basically full of air at idle (no restriction to air flow), this type of engine at idle has superior fuel consumption characteristics. This is why most operators of diesel trucks - once they are started - do not switch off for the remainder of the day, they are so efficient.

However, because they have a narrow rev range and a low ability to gain rpm (that's horsepower, by the way) they are not as efficient as a gas engine at higher speeds because the fuel is slow to burn. Under full load conditions, because of the shorter combustion process at higher revs, quite a bit of exhaust does not get burnt. This results in that sooty exhaust that can be seen trailing behind our buses, especially if they are going uphill.

New technology has negated some of these disadvantages, but some are band-aid fixes to make the car more appealing to consumers. They are also harder to start in cold weather (not much of a problem in Jamaica) and diesel cars are viewed as less powerful.

Common Rail Diesel injection has addressed the unburnt fuel issue by metering the oil through computer-controlled injectors. This technique has cleaned up the exhaust considerably but unburnt fuel is still an issue when it comes on to higher speed motoring.

combustion

To lessen piston slap caused by the high shock wave produced by diesel combustion, some manufacturers employ a 'pre-chamber' in which the burn is initiated, and then 'introduced' into the main combustion chamber. This lessens the pressure pulse, and thus reduces diesel's characteristic clatter. But it also negates some of the efficiencies that people buy a diesel for in the first place.

Turbochargers (exhaust driven superchargers) expand the rev range of the diesel, and give it some well-needed poke. This technology actually works, and because the exhaust temps are cooler (more efficient) diesel turbos tend to live a longer life than those attatched to their gasolene counterparts.

Performance diesel engines based on turbo technology have been competitive to the point of winning races. Audi diesels have one, the 24 Heures de Le Mans, the world's most prestigious endurance race - albeit with certain rules slanted in their favour. According to the Fédération Internationale de l'Automobile, the rules were relaxed so that diesels were allowed to run more displacement, more intake area and more boost.

The Achilles heel for diesels, though, will always be the pollutants they produce and their environmental impact. Because of the higher temperatures during the power phase, more oxides of nitrogen are produced than is the case for gasolene engines.

non-profit organisation

According to the Clean Air Task Force, a non-profit organisation based in Boston, United States of America, "Diesel engine exhaust contains a number of a potent carcinogens-particulate matter (largely elemental and organic carbon soot) coated in gaseous organic substances such as formaldehyde and PAH (a group of super-toxic gases that attach themselves to particles), shown to result in adverse birth outcomes in Polish children, and respiratory irritants, such as acrolein."

For further reseacrh see, the website http://www.catf.us/publications/reports/Diesel_in_America_Technical_Paper.pdf.

Motorists who own trucks like this 6.4-litre, 350-horsepower diesel engine are spending much more on diesel fuel.- Contributed