Modern Diesel Engines

Unlike petrol engines, diesel engines don’t need ignition system. Due to the inherent property of diesel, combustion will be automatically effective under a certain pressure and temperature combination during the compression phase of Otto cycle. Normally this requires a high compression ratio around 22 : 1 for normally aspirated engines. A strong thus heavy block and head is required to cope with the pressure. Therefore diesel engines are always much heavier than petrol equivalent.

The lack of ignition system simplifies repair and maintenance, the absence of throttle also help. The output of a diesel engine is controlled simply by the amount of fuel injected, this makes the injection system very decisive to fuel economy. Common rail direct injection system, gifted by its high-pressure, precise injection, improves fuel efficiency a lot.

Even without direct injection, diesel inherently delivers superior fuel economy because of leaner mixture of fuel and air. Unlike petrol, it can combust under very lean mixture. This inevitably reduce power output (no free lunch !), but under light load or partial load where we don’t need so much power, its superior fuel economy shines.

Another explanation for the inferior power output is the extra high compression ratio. On one hand the high pressure and the heavy pistons prevent it from revving as high as petrol engine (most diesel engine deliver peak power at lower than 4500 rpm.), on the other hand the long stroke dimension required by high compression ratio favours torque instead of power. This is why diesel engines always low on power but strong on torque.

To solve this problem, diesel makers prefer to add turbocharger. Turbo’s top end power suits the torque curve of diesel very much, unlike petrol. Therefore today’s turbo diesel output similar power to a petrol engine with similar capacity, while delivering superior low end torque and fuel economy. For instance :
 

Model	Power	Torque	Top speed	0-60mph	0-100mph	30-70mph	50-70mph (top gear)	Fuel  consumption
Audi A3 1.8 Sport	125hp	127lbft	122mph	9.8sec	30.2sec	10.1sec	10.9sec	35mpg
Audi A3 1.9 TDi Sport	110hp	166lbft	119mph	9.7sec	30.7sec	9.4sec	8.6sec	54mpg

 
As emission regulations keep tightening in Europe, as diesel technology is progressing and catching up petrol, European car makers produce more and more diesel engines. Today diesel consists of 1/4 to 1/3 cars sold over there. Some countries like France and Italy the percentage is even up to 40%. In Germany, Mercedes’ engineers expressed their worry about the tightening of emission regulations in the future may eventually kill all large capacity petrol engines, say, V8 and V12. They believe diesel is the only way to pass the requirements. Not only Stuttgart, but BMW and Audi have also developed their first-ever powerful turbo diesel V8s for fitting in their flagship models.

Diesel technology is taking off. The last problem to be cleared is the excessive particles emitted, which is mostly carbon or large hydrocarbon particles contributing to smog and dark smoke. PSA has developed a particle filter and will be equipped to its HDi common-rail series in year 2000. Hopefully it will bring even brighter future for diesel as well as our environment.

In the US, where petrol is cheaper than bottled water, virtually no one is producing diesel cars. Instead, they put their bet on fuel cell technology. (Ford will put fuel car cars into mass production in 2004) However, most experts agree that fuel cell won’t be able to replace conventional combustion engines in the foreseeable future. Technology breakthrough in fuel cell does not come as big and as quick as diesel.

Common-Rail Direct Injection

Compare with petrol, diesel is the lower quality ingredient of petroleum family. Diesel particles are larger and heavier than petrol, thus more difficult to pulverise. Imperfect pulverisation leads to more unburnt particles, hence more pollutant, lower fuel efficiency and less power. Common-rail technology is intended to improve the pulverisation process.

To improve pulverisation, the fuel must be injected at a very high pressure, so high that normal fuel injectors cannot achieve. In common-rail system, the fuel pressure is implemented by a strong pump instead of fuel injectors. The high-pressure fuel is fed to individual fuel injectors via a common rigid pipe (hence the name of "common-rail"). In the current first generation design, the pipe withstand the pressure as high as 1,350 bar or 20,000 psi. Fuel always remains under such pressure even in stand-by state. Therefore whenever the injector (which acts as a valve rather than a pressure generator) opens, the high-pressure fuel can be injected into combustion chamber quickly. As a result, not only pulverisation is improved by the higher fuel pressure, but the duration of fuel injection can be shortened and the timing can be precisely controlled.

Benefited by the precise timing, common-rail injection system can introduce a "post-combustion", which injects small amount of fuel during the expansion phase thus create a small scale combustion before the normal combustion takes place. What’s the purpose ? This further eliminate the unburnt particles, also increase the exhaust flow temperature thus reduce the pre-heat time of the catalytic converter. In short, "post-combustion" cuts pollutants.

How effective is it? According to PSA's press release, its new common-rail engine (in addition to other improvement) cuts fuel consumption by 20%, doubles torque at low engine speeds and increases power by 25%. It also brings a significant reduction in the noise and vibrations of conventional diesel engines. In emission, greenhouse gases (CO2) is reduced by 20%. At a constant level of NOx, carbon monoxide (CO) emissions are reduced by 40%, unburnt hydrocarbons (HC) by 50%, and particle emissions by 60%.

PSA's Particle Filter (PF)

Since the '80s particle emissions from cars have been reduced by three-quarters, thanks to the Government’s legislation as well as the advances in diesel technology such as direct injection. However, diesel engines still emit more particles than petrol, and it seems that it is the nature of diesel. Therefore the largest diesel car maker, PSA, developed a particle filter for use in next year’s (2000) HDi common-rail diesel family.

Basically, PSA’s particle filter (PF) is a porous silicon carbide unit, comprising passageways which has a property easily trap and retain particles from the exhaust gas flow. Before the filter surface is fully occupied, these carbon / hydrocarbon particles should be burnt up, becoming CO2 and water and leave the filter accompany with exhaust gas flow. We call this process as regeneration.

Normally regeneration takes place at 550° C. However, the main problem is: this temperature is not obtainable under normal conditions. PSA tells us normally the temperature varies between 150° and 200°C when the car is driving in town, as the exhaust gas is not in full flow.

Luckily, the new common-rail injection technology helps solving this problem. Gifted by its high-pressure, precise injection during a very short period, the common-rail system can introduce a "post-combustion" by injecting small amount of fuel during expansion phase. This increases the exhaust flow temperature to around 350°C.

Then, a specially designed oxidising catalyst converter locating near the entrance of the particle filter unit will combust the remaining unburnt fuel come from the "post-combustion". This raise the temperature further to 450° C.

The last 100°C required is fulfilled by adding an addictive called Eolys to the fuel. Eolys lowers the operating temperature of particle burning to 450° C, now regeneration occurs. The liquid-state additive is store in a small tank and added to the fuel by pump. The PF unit needs to be cleaned at dealerships every 80,000 km by high-pressure water, to get rid of the deposits resulting from the additive.

One more thing to be solved is the influence of "post-combustion". It increases engine torque when the driver doesn’t expect. Therefore the engine management system has to regulate the torque by adjusting the amount of normal fuel injection, pre-injection etc. and turbocharger’s boost pressure to compensate.