The basics of diesel engine technology, and the practicalities of driving a diesel car in Singapore
The ABCs of 'D' PHOTO: TORQUE

While both petrol and diesel engines are essentially reciprocating-piston,
internal combustion machines, the fuels in question aren’t interchangeable because there is a fundamental difference in the way the combustion is initiated. For this reason, the two engine types are designed and constructed differently. Both, however, have powered automobiles, in one form or another, for over a century.

During much of those 100-odd years, diesel was always the poorer cousin of petrol – being the preferred propellent for trucks, buses, taxis and locomotives. Diesel fuel was much cheaper than petrol, too, while diesel-fuelled engines were noisy, smoky and slow, earning them a decidedly working-class reputation.

Well, the world is no longer the same, and technology has changed for the better.

SAME STROKES, DIFFERENT STOKES


Like a typical petrol engine, a diesel engine follows the 4-stroke principle of reciprocating pistons – intake, compression, ignition, exhaust. As such, the crankshaft, pistons, valves, camshaft and injectors are all present.
Conspicuously absent, however, is the spark plug, because in a diesel engine, ignition to initiate combustion occurs when the fuel is injected into highly compressed air. This is why it is also called a compression-ignition (CI) engine.

The air-fuel mixture in a petrol (or spark-ignition) engine is maintained at a fixed ratio (roughly 14:1 by mass). It is this mixture that's ignited by the spark plug near the end of the piston’s compression stroke. In a diesel engine, however, only air is compressed inside the cylinder, but to a pressure four times higher, achieved by the high compression ratio.

What results is a chamber containing air pressurised to about 80 bar (80 times the atmospheric pressure around us) and with a temperature of at least 700 deg C, both of which are conditions ideal for diesel fuel to self-ignite without the need for a spark. And this is exactly what happens when the fuel is injected in a fi ne spray at extremely high pressure (which creates that familiar diesel clatter). There is no throttle on the intake manifold, and power delivery is solely a function of fuel quantity injected.


ALL TORQUE, ALL ACTION


The biggest revelation to motorists who only “know” petrol would be the modern turbo-diesel car's astonishing mid-range acceleration. Terrific torque at low-to-medium engine revs is the secret here, and it makes city driving surprisingly shiok.

Even the most highly tuned diesel engines rarely rev past 5000rpm, with the vast majority of today's diesel passenger cars developing their maximum energy at around 4000rpm. But with massive amounts of torque appearing very early in the engine speed range, a good turbo-dieselcar doesn’t need to rev high.

It's very efficient, too, by internal combustion standards. High compression, usually more than 15:1, plays a key role in this. With turbocharging, the increased effective compression ratio in a diesel
engine leads to even greater expansion of gases in the combustion chamber.

In other words, more work is done, and since Newton-metre (Nm) is the measure of said work, the rotating crankshaft ultimately develops more torque thanks to the turbo (or turbos, if the engine is equipped with more than one).

While the specific power output is limited by a diesel plant's relatively low
engine speeds, a typical 2-litre turbodiesel's peak torque output easily matches or betters that of a turbo petrol 2-litre. For instance, Mercedes' 3-litre turbo-diesel V6 generates 619Nm, which compares favourably to the 618Nm produced by the automaker's 6.2-litre AMG V8!

ALWAYS WORKING HARD


All that work done by the turbo-diesel means the hardware has to be able to cope. High compression, high pressures, high temperatures and "spontaneous" ignition require a reinforced engine block, stronger valvetrains and a more robust cylinder head. The reliability and durability of diesel is also "helped" by its history of "heavy duty" use in commercial vehicles.

Today, the diesel clatter continues to be heard from compression-ignition
engines, but "behind" the unsophisticated noise is advanced technology such as turbocharging, ultra-high pressure injection (1700-2000 bar), high-precision fuel metering, catalytic converters, soot filters and computer-controlled engine management. These devices and systems
have made the modern diesel engine powerful, economical and, believe it or not, even desirable.

'D' DRIVE


With the implementation of CEVS (Carbon Emissions-based Vehicle
Scheme) and the reduction of diesel’s so-called special tax,
passenger cars with a “D” suffi x have become feasible alternatives
to their traditional petrol-powered equivalents. Even though it still attracts a higher annual road tax than a petrol car of the same
engine displacement, the diesel car's significantly lower fuel consumption would allow the owner to break even every year, so to speak. With serious mileage clocked (more than 15,000km annually), the diesel will even offer an advantage in running cost.

What about maintenance? Well, there was a time when diesel engines in motorcars required an oil change every 1,000km. They also needed special engine lubes and frequent replacement of the fuel filter. These days, however, the newest Euro 5 and Euro 6 diesel vehicles are designed to be treated no differently from their petrolfuelled counterparts.

Both Kia and Hyundai, for example, recommend nothing more than a specific grade of synthetic engine lubricant. In the taxi business, Hyundai Sonatas cover about 100,000km a year and are serviced every 10,000km – which is exactly the same as for privately owned Korean runabouts. Over at Volkswagen, the recommended service interval for its TDI passenger cars is 15,000km – which is, again, the same as for TSI petrol VWs. The Germans even specify the exact same engine oil for their diesel and petrol powerplants.

In a diesel car, there isn't an ignition system to worry about, and of course, no spark plugs to clean or replace. Perhaps the only item that requires attention is the diesel particulate filter (DPF), which is incorporated in the exhaust system of diesel engines that comply with Euro 5 or "higher" standards. The soot that builds up in the DPF needs to be burnt off periodically, in a process termed "regeneration". For this
to take place, the engine has to operate at relatively high speeds.
So, if your diesel car spends all its time in city traffic, driven at low speeds, it would be necessary to take it for a quick highway run once in a while.