Feature
Diesel Do Nicely
Why Americans should fall in love with the diesel
Here are some figures to conjure with:
Consider those for a moment. The diesel-powered Audi A8 4.2 TDI is not only more economical than the gasoline-powered A8 4.2, it also produces fewer greenhouse gases and, most importantly for readers of this Web site, it's also significantly quicker.
For the past few years, while the U.S. has been bemoaning ever increasing gas prices, the Europeans have been experiencing a quiet, bloodless diesel revolution. The introduction of multivalve, common-rail direct-injection and pre-ignition technology has transformed the diesel engine from a noisy agricultural workhorse into an ultrarefined, economical powerhouse worthy of the finest performance and luxury cars. Across Europe, more than a third of new cars now stop at the black pump and in some countries, diesel cars outsell petrol.
It's a staggering transformation, and one from which the U.S. could learn some important lessons.
The techie bit
The diesel revolution in Europe has been driven by new technology. Here are some of the headlining acts:
Multivalve technology — Most modern turbodiesels, such as the Audi V8, feature four valves per cylinder — two inlet and two exhaust. This allows the fuel-injection nozzle to be positioned in the center of the combustion chamber, producing more efficient, symmetrical combustion. Power is increased, while harmful emissions are reduced.
Computer-controlled direct injection — The car's onboard computer takes readings from a range of sensors, such as the throttle position and engine speed, and carefully regulates the fuel supply, which is injected directly into the combustion chamber. Diesels do not require spark plugs; the fuel ignites automatically when mixed with hot air.
The Audi V8 TDI is one of the first engines to use Piezo injectors. These are capable of five separate injections of fuel during each engine stroke — at 4,000 rpm that's 20,000 injections per minute or 333 per second. The efficiency of the combustion is greatly improved.
Common-rail technology — Diesels of old had separate fuel lines from the pump to each injector. The pressure and injection timing were dependant on the engine speed. Modern common-rail systems, by contrast, unite all the injectors on a single feed line and operate independently of engine speed. The fuel supply can therefore be precisely managed by the onboard computers. Common-rail systems also operate at much higher injection pressures, which helps optimize the fuel-air mix and the efficiency of the combustion process.
Pilot injection — Sometimes called "pre-ignition," pilot injection eliminates the combustion spikes that cause the "rattle" traditionally associated with diesel engines, especially at idling speeds. Pilot injection introduces a small quantity of fuel into the combustion chamber prior to the main power-inducing explosion. This injection takes place within a few ten-thousandths of a second of the main explosion and results in a smoother combustion cycle and reduced clatter. Even at a standstill, an Audi, BMW or Mercedes-Benz diesel is now all but as refined as its gasoline sibling.
Pilot injection has two other significant advantages. The system enhances the engine's cold-start capability, so that diesels can now function at temperatures as low as minus 40 degrees Fahrenheit. Pilot injection also helps reduce nitrous-oxide emissions by lowering peak combustion temperatures.
Turbocharging — A turbodiesel is hardly a new concept, but the blowers used are becoming increasingly sophisticated. The Audi V8 TDI features twin turbochargers and twin intercoolers. BMW's new 535d, which is actually a 3.0-liter, is the first production diesel to use sequential turbos. A smaller turbo operates from idle to low engine speeds — helping to generate 368 lb-ft at 1,500 rpm — before a second, larger turbo takes over at higher engine speeds. The net result is 272 hp and 413 lb-ft of torque.
Emissions — Low-sulphur diesel, which becomes mandatory in the U.S. next year, has been available in Europe for some time. As well as eliminating unpleasant odors, low-sulphur fuel enables the use of catalytic converters and particulate filter traps to dramatically reduce the nitrous oxides and other harmful emissions produced by diesel engines. The Audi V8 combines twin catalytic converters with a particular filter trap that requires no additives and will therefore not need servicing during the car's lifespan.
The road to Damascus
In years gone by, the case for diesel was made on rational, economic grounds. Diesels might have been fractionally more expensive to buy, but the additional outlay was soon offset by reduced running costs and higher residual values. So what if it rattled a bit when cold, you could always turn up the stereo.
Today, the economic rationale in favor of diesel is still relevant, but it's been joined by more emotive impulses. In the U.K., for example, the BMW 530i and 530d cost similar money. They have the same power output (231 hp), but the diesel has dramatically more torque — 212 lb-ft vs. 384 lb-ft. The 530i gets to 62 mph 1.3 seconds quicker than the 530d (which takes 7.8 seconds), but in the midrange, when the torque becomes more relevant, the diesel car pulverizes its petrol rival. That it's astonishingly refined, even at idle, and achieves an average of 42.2 mpg to the 530i's 32.1 mpg (European "combined" estimates for manual-transmission vehicles), is really the cherry on top of the icing on top of the cake. The 530d is the enthusiast's choice, pure and simple.
The case for the opposition
Some performance-car manufacturers argue that diesel power runs counter to their brand values. Porsche, for example, reckons that the relatively low-revving characteristics of a diesel engine are ill-suited to a performance application. "A Porsche engine must be free revving," argues Jan Roth, the project chief for the new Cayman S. "The Cayman engine's rev limiter is set at 7,300 rpm; most diesels produce their peak power around 4,500 rpm."
Porsche is also concerned about the noise of the engine. While the Audi V8 emits a throaty, deep growl, no diesel will match the sonorous roar of a Porsche boxer. Instead of diesel, the Weissach boffins are trying to solve the consumption/emissions conundrum by developing a hybrid version of the Cayenne, which will be launched before the end of the decade.
Diesel engines also tend to be heavier than their petrol counterparts. In a front-engine car, placing so much mass over the front wheels can compromise the car's balance and handling. The Volkswagen Touareg 5.0 V10 TDI, for example, suffers from excessive understeer.
Final thoughts
While diesel clearly isn't the answer to everyone's prayers, the U.S. market is unquestionably missing out on the modern diesel phenomenon. Bountiful torque, excellent refinement and a huge range are qualities well suited to the American highway. It is surely time to put away the prejudices of the 1970s and embrace the modern diesel engine.
Audi A8 4.2 TDI quattro | Audi A8 4.2 quattro | |
| Engine | 326 - horsepower V8 | 335 - horsepower V8 |
| Torque (lb-ft) | 479 | 317 |
| 0-100 km/h (0-62 mph) | 5.9 | 6.2 |
| Top speed (mpg) | 155 | 155 |
| Average consumption (mpg) | 30.0 | 23.7 |
| Range (miles) | 594 | 469 |
| Emissions (CO2 g/km) | 253 | 286 |
Consider those for a moment. The diesel-powered Audi A8 4.2 TDI is not only more economical than the gasoline-powered A8 4.2, it also produces fewer greenhouse gases and, most importantly for readers of this Web site, it's also significantly quicker.
For the past few years, while the U.S. has been bemoaning ever increasing gas prices, the Europeans have been experiencing a quiet, bloodless diesel revolution. The introduction of multivalve, common-rail direct-injection and pre-ignition technology has transformed the diesel engine from a noisy agricultural workhorse into an ultrarefined, economical powerhouse worthy of the finest performance and luxury cars. Across Europe, more than a third of new cars now stop at the black pump and in some countries, diesel cars outsell petrol.
It's a staggering transformation, and one from which the U.S. could learn some important lessons.
The techie bit
The diesel revolution in Europe has been driven by new technology. Here are some of the headlining acts:
Multivalve technology — Most modern turbodiesels, such as the Audi V8, feature four valves per cylinder — two inlet and two exhaust. This allows the fuel-injection nozzle to be positioned in the center of the combustion chamber, producing more efficient, symmetrical combustion. Power is increased, while harmful emissions are reduced.
Computer-controlled direct injection — The car's onboard computer takes readings from a range of sensors, such as the throttle position and engine speed, and carefully regulates the fuel supply, which is injected directly into the combustion chamber. Diesels do not require spark plugs; the fuel ignites automatically when mixed with hot air.
The Audi V8 TDI is one of the first engines to use Piezo injectors. These are capable of five separate injections of fuel during each engine stroke — at 4,000 rpm that's 20,000 injections per minute or 333 per second. The efficiency of the combustion is greatly improved.
Common-rail technology — Diesels of old had separate fuel lines from the pump to each injector. The pressure and injection timing were dependant on the engine speed. Modern common-rail systems, by contrast, unite all the injectors on a single feed line and operate independently of engine speed. The fuel supply can therefore be precisely managed by the onboard computers. Common-rail systems also operate at much higher injection pressures, which helps optimize the fuel-air mix and the efficiency of the combustion process.
Pilot injection — Sometimes called "pre-ignition," pilot injection eliminates the combustion spikes that cause the "rattle" traditionally associated with diesel engines, especially at idling speeds. Pilot injection introduces a small quantity of fuel into the combustion chamber prior to the main power-inducing explosion. This injection takes place within a few ten-thousandths of a second of the main explosion and results in a smoother combustion cycle and reduced clatter. Even at a standstill, an Audi, BMW or Mercedes-Benz diesel is now all but as refined as its gasoline sibling.
Pilot injection has two other significant advantages. The system enhances the engine's cold-start capability, so that diesels can now function at temperatures as low as minus 40 degrees Fahrenheit. Pilot injection also helps reduce nitrous-oxide emissions by lowering peak combustion temperatures.
Turbocharging — A turbodiesel is hardly a new concept, but the blowers used are becoming increasingly sophisticated. The Audi V8 TDI features twin turbochargers and twin intercoolers. BMW's new 535d, which is actually a 3.0-liter, is the first production diesel to use sequential turbos. A smaller turbo operates from idle to low engine speeds — helping to generate 368 lb-ft at 1,500 rpm — before a second, larger turbo takes over at higher engine speeds. The net result is 272 hp and 413 lb-ft of torque.
Emissions — Low-sulphur diesel, which becomes mandatory in the U.S. next year, has been available in Europe for some time. As well as eliminating unpleasant odors, low-sulphur fuel enables the use of catalytic converters and particulate filter traps to dramatically reduce the nitrous oxides and other harmful emissions produced by diesel engines. The Audi V8 combines twin catalytic converters with a particular filter trap that requires no additives and will therefore not need servicing during the car's lifespan.
The road to Damascus
In years gone by, the case for diesel was made on rational, economic grounds. Diesels might have been fractionally more expensive to buy, but the additional outlay was soon offset by reduced running costs and higher residual values. So what if it rattled a bit when cold, you could always turn up the stereo.
Today, the economic rationale in favor of diesel is still relevant, but it's been joined by more emotive impulses. In the U.K., for example, the BMW 530i and 530d cost similar money. They have the same power output (231 hp), but the diesel has dramatically more torque — 212 lb-ft vs. 384 lb-ft. The 530i gets to 62 mph 1.3 seconds quicker than the 530d (which takes 7.8 seconds), but in the midrange, when the torque becomes more relevant, the diesel car pulverizes its petrol rival. That it's astonishingly refined, even at idle, and achieves an average of 42.2 mpg to the 530i's 32.1 mpg (European "combined" estimates for manual-transmission vehicles), is really the cherry on top of the icing on top of the cake. The 530d is the enthusiast's choice, pure and simple.
The case for the opposition
Some performance-car manufacturers argue that diesel power runs counter to their brand values. Porsche, for example, reckons that the relatively low-revving characteristics of a diesel engine are ill-suited to a performance application. "A Porsche engine must be free revving," argues Jan Roth, the project chief for the new Cayman S. "The Cayman engine's rev limiter is set at 7,300 rpm; most diesels produce their peak power around 4,500 rpm."
Porsche is also concerned about the noise of the engine. While the Audi V8 emits a throaty, deep growl, no diesel will match the sonorous roar of a Porsche boxer. Instead of diesel, the Weissach boffins are trying to solve the consumption/emissions conundrum by developing a hybrid version of the Cayenne, which will be launched before the end of the decade.
Diesel engines also tend to be heavier than their petrol counterparts. In a front-engine car, placing so much mass over the front wheels can compromise the car's balance and handling. The Volkswagen Touareg 5.0 V10 TDI, for example, suffers from excessive understeer.
Final thoughts
While diesel clearly isn't the answer to everyone's prayers, the U.S. market is unquestionably missing out on the modern diesel phenomenon. Bountiful torque, excellent refinement and a huge range are qualities well suited to the American highway. It is surely time to put away the prejudices of the 1970s and embrace the modern diesel engine.
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