BACKGROUND
Australia’s long history of relatively cheap petrol has seen passenger car diesels languishing in the backwater with appeal limited to those rural owners whose huge distances and easy access to diesel fuel could justify a big premium in purchase price and loss of performance. In Australian cities, LPG has been the alternative fuel of choice for high mileage fleets and taxis, not diesel.
This is about to change as new requirements applicable from January 1, 2006 dictate a lift in the quality of diesel fuel that brings Australia into line with stringent European requirements. The local market can now benefit from new diesel technology that delivers radical power increases as well as dramatic reductions in pollution and fuel consumption. Just as European markets have embraced these advances for some years, it will change the way Australians look at diesels.
These changes are tempered only by the 10 per cent price premium that diesel attracts over basic unleaded petrol in some areas of Australia. Diesel should be cheaper than petrol however local refining and global supply issues can change that relativity.
Because diesel oil contains more energy per litre than petrol, an efficient diesel engine should go much further on the same amount of fuel. The generally accepted relativity is 30 per cent further on the same quantity of fuel with a 30 per cent loss of specific power (kW/litre), all else being equal.
One of the most graphic comparisons of older diesel/petrol relativities was the 1986 Mazda 626 which offered diesel and petrol variations of the same 2.0-litre engine. The diesel produced 47kW/4650rpm and 120Nm/2750 on a compression ratio of 22.7:1. Its petrol equivalent delivered 68kW/5000rpm and 150Nm/3500rpm on a compression ratio of 8.6:1. However, the diesel’s highway fuel figure was just 5.0lt/100km compared to 7.8 for the petrol engine. Around town, the diesel used just 7.0lt/100km against the petrol engine’s 11.0lt/100km for a huge saving.
Diesel engines then lost most of their fuel economy advantages in the 1990s after advanced electronics were applied to petrol engines and new premium unleaded fuels allowed a return to high-compression petrol engines. As the same advances are applied to diesel engines in conjunction with the arrival of cleaner diesel fuels, the gap is set to widen again.
Look at the advances in, for example, the light commercial market. Prior to 2006, the Mazda 2.5-litre turbodiesel was typical with its relatively feeble 82kW and 271Nm. Nissan's new Euro-spec diesel launched late in 2005 in the latest Navara delivers 128kW and 403Nm from the same capacity with a 10-20 per cent improvement in fuel economy.
Compare this to Subaru's latest 2.5-litre petrol engine with its 121kW and 226Nm. Even if both diesels have turbochargers and the base Subaru engine doesn’t, the loss of performance is no longer a diesel handicap.
This changes the whole ownership formula from a resale perspective. Previously, a dozy used diesel passenger car could prove difficult to sell to a limited market. The combination of its higher initial purchase price and lower resale could soon negate any advantages at the bowser.
Today’s developments and a steady increase in fuel prices should soon turn this around to where a diesel passenger car should hold its new price premium from showroom to the recycling yard.
There are already precedents in the 4X4 SUV and light commercial market where even the old technology diesels are more sought after than their petrol equivalents.
WHAT IS A DIESEL?
This is a fair question when petrol and diesel engines look the same and share most internal components.
The fundamental difference is that a petrol engine runs much lower compression and therefore relies on a spark from an ignition system to ignite the fuel-air mixture. Because a diesel engine generates compression up to two to three times greater it creates so much internal heat that the fuel will spontaneously ignite without a spark. (Ed: as such diesels are sometimes known as compression ignition engines.)
Because it takes anything up to 20 minutes for any engine to warm-up fully, diesel engines usually only make more sense over trips long enough to generate sufficient heat for complete and efficient combustion. Diesels do not deliver their best during short-haul stop-start runs where the engine is shut down before reaching operating temperature. They also deliver higher pollution levels under these conditions which generate extra maintenance... What's all this mean to you? Diesels are not for everyone.
THERE IS NO SPARK PLUG, SO WHAT?
There's more... A diesel engine compresses air when a petrol engine compresses a mixture of fuel and air. A diesel engine therefore relies on a fuel-injection system that can inject the correct amount of fuel directly into the cylinder just prior to combustion. Injectors that can survive the combustion process supplied by a precision pump that can maintain the correct fuel pressure and quantity for this to happen at exactly the right moment are critical to the diesel process.
In a traditional diesel engine design, electric glow plugs are required to introduce heat into each cylinder before the engine will start. Think of them as tiny electric radiators inside each cylinder which is how they work. This can dictate a wait for up to a minute before you operate the starter motor. At this point, the diesel engine usually generates heavy levels of pollution as well as a huge drain on the battery. Nowadays as diesel engine manufacturers introduce powerful electronics to the fuel distribution process, the timing and quantity of fuel entering each cylinder can be controlled so finely that glow plugs and the starting delay are eliminated in most modern designs.
IF THEY ARE SO SIMPLE, WHY DO DIESELS COST SO MUCH MORE?
The theory might be fine but there is always a bottom line. In reality, a diesel engine, like a high-compression petrol engine, generates extra stresses that dictate substantially upgraded parts including the engine block in some cases. The forces on pistons, piston rings, gudgeons, conrods, bearings and crankshaft rise in direct proportion with the two to threefold increase in compression.
The injectors, injection pump and glow plugs where fitted, are all high-precision, long-life and therefore expensive parts. A diesel engine requires a beefy starter motor to turn it over which in turn dictates a bigger battery and alternator. Heavy-duty cooling systems are required to keep a diesel operating within its optimum operating band in all conditions.
A diesel engine is far more sensitive to dirt and water in the fuel so air and fuel filters are substantially upgraded. Some have separate water traps to isolate the water from the fuel. Because of the soot and other residues generated by diesel fuel, diesel engines require special oils for engine lubrication and filters that can cost two to three times those for a petrol engine.
All of this generates extra weight. Along with the extra noise and vibration inherent in a diesel, most diesel installations require extra sound absorption measures, more complex and beefier engine mounts and stronger suspension. The extra torque (or twisting power) inherent in a diesel can also dictate extra body reinforcement, stronger transmissions, axles, clutches and other driveline components. When most of these parts are beefed-up anyway in a commercial or heavy-duty 4WD vehicle, the impact of a diesel engine is not as pronounced.
As strong new lightweight materials are developed, many of these issues are now addressed at the source which is already cutting back the gap in price. Because a diesel operates most efficiently at lower engine speeds and generates strong exhaust pressure, most modern diesels are fitted with a turbocharger to close the power gap. Additional cooling and lubrication requirements for the turbocharger plus the turbocharger and intercooler themselves all add their own costs similar to a turbocharged petrol engine.
ARE THERE HIDDEN COSTS?
Not really, providing a diesel performs most of its service at full operating temperature.
They can prove a nightmare in short trips. Glow plugs can fail. Expensive engine oil and filters can require more frequent changing as they become saturated in soot. Think of the soot that a pressure lantern produces before it gets hot and burns a clean white light. A diesel engine operates along similar principles when it requires pressure and heat for complete combustion.
All filters and oils are more expensive, some by a factor of two or three. These can become a significant extra cost if usage dictates more frequent changes.
Fuel quality is a real issue as most diesel engines also require the fuel to lubricate expensive fuel system components. If the fuel is missing the required additives, failures result very quickly. The exact mix of diesel fuel is highly sensitive to temperature as some brews create wax deposits that block the fuel system in a cold snap. (Ed: this rarely affects Aussie diesels, however, some care may need to be taken in true alpine conditions)
The latest engines require highly-refined low sulphur fuels and may not tolerate even a tankful of the “dirty” old style diesel. Before buying any used diesel, it is worth checking the consistency of your fuel supply and see if it matches the manufacturer’s requirements.<.P>
IS DIESEL A GREEN FUEL?
It is and it isn’t...
There was a push towards diesel when it didn’t produce some of the chemical nasties but most of these have since been all but eliminated in moden petrol engines anyway. A more insidious byproduct of diesel engines has since reared its ugly head as they have become more popular. Diesel exhaust contains fine particles which can aggravate asthma and cause lung damage as well as premature death.
Many global environmental authorities have classified diesel particulate matter as a likely human carcinogen. Under bright Australian sunlight, these diesel particulates are of even greater concern as a trigger for photochemical smog in our big cities.
The 2006 push to clean up diesel fuel and thus open the door for the latest clean-running diesels is timely when Australian diesel numbers are approaching levels where particulate emissions could become a problem. They are already a concern in areas where heavy transport is concentrated.
Most used diesel passenger cars in Australia are of the old school and don’t reflect the latest clean-up measures when fuel quality was not always compatible with the latest designs.
Diesel engines can run on a number of bio-fuels and this may prove to be an advantage in the future.
WHAT ELSE DO I NEED TO KNOW?
Much is made of how unpleasant filling a diesel fuel tank can be. Where spilt petrol evaporates, diesel can leave an oily residue behind. The biggest issue is never, ever fill the tank with petrol!
Driving a diesel manual can require a different technique. If anything it should be easier for most drivers. A diesel’s high compression and low revs mean you change up far earlier than in a petrol engine. If it has a turbo, there is usually a slight lag from rest but once on the move the turbo should keep spinning and eliminate this lag providing you change at the right engine speed.
Drivers of automatic cars won’t notice any real difference.
The diesel engine is the best revenge for the speed camera. Its high compression means you have to consciously accelerate and hold a certain speed and it won’t run away from you, even downhill. In fact, this is one of the most pleasing aspects of a modern diesel engine: at a time when more petrol engines keep on revving even with your foot off the accelerator, the diesel just drops its revs as soon you lift your foot. This powerful engine braking effect should cut brake wear.
A diesel’s low speed torque should also increase clutch life dramatically when most diesel engines can trickle along happily at unbelievably low revs.
Because a diesel’s high compression and precision fuel pump and injectors are fundamental to smooth, clean running, they are more important than in a petrol engine. Both are expensive to rectify so a good oil and filter change history is more important than for a petrol engine. The high compression will also quickly find any weaknesses in the cooling system including head gasket so coolant history is also more critical.
USED PASSENGER CAR DIESELS IN AUSTRALIA
Audi
Audi offered a relatively efficient 2.5-litre 132kW twin-turbo V6 diesel in its Allroad Quattro TDI wagon. The Audi A6 came with a 3.0-litre V6 160kW/450Nm diesel option from late 2004.
BMW
The X5 wagon came with a 135kW/390Nm 3.0-litre inline six-cylinder diesel in 2003 that was upgraded to 150kW/480Nm in 2004.
Citroen
The medium-sized Xantia liftback came as a 2.0-litre turbodiesel with 67kW/196Nm during 1996-98. Its larger C5 replacement came as the HDi sedan and wagon upgraded to 82kW/259Nm from 2001-05.
Holden
The original rear-drive Gemini sedan came as a useful diesel model from 1981-84 as a TE, TF and TG series. Its 1817cc non-turbo diesel delivered 40kW and 105Nm.
Mazda
Mazda found a steady but tiny rural market with its 626 diesel sold from 1984-87. Its 2.0-litre capacity delivered 47kW/120Nm.
Mercedes-Benz
Mercedes-Benz established a strong local diesel presence with early 240D versions of its compact range, the backbone of European taxis at the time. These evolved into the W123 240D (53kW/137Nm) and five-cylinder 300D (65kW/172Nm) from 1980. An advanced new 300D (80kW/185Nm) inline six cylinder was released as part of the W124 range that replaced the W123 series in 1986. There were wagon versions in both W123 and W124 series.
After the W124 evolved into the E-Class, the diesel was re-launched as the E300D with 100kW/210Nm for 1993-94. It was replaced by the E300D Turbo in the new W210 series from 1997-99 with boosts to 130kW/330Nm. It retained the same inline six-cylinder 3.0-litre configuration from 1986.
The E300D Turbo was then replaced by the next generation diesel but in the same W210 body in 1999. This new E270 CDI was a turbo five-cylinder design of 2.7-litre with 125kW/400Nm and ran from 1999-2002.
The most recent E270 CDI received an upgraded version of this five-cylinder engine with 130kW/425Nm from 2002-05 during which it was fitted to the current W211 E-Class body. It was replaced late in 2005 with a 3.0-litre V6 engine in the E280 CDI delivering 140kW/400Nm but still based on the W211 body series.
The new baby W201 series was available as the 190D 2.5 from 1986-88 with a five-cylinder 2.5-litre diesel delivering 66kW/154Nm before this small Mercedes-Benz model was renamed the C-Class. This level of diesel was revived in the current W203 series as the C220 CDI in 2001 with a new generation turbo four-cylinder diesel of 2.1-litre delivering 105kW/315Nm.
Peugeot
Like Mercedes-Benz and Volkswagen (see below), Peugeot built a steady local diesel following based on its earlier models such as the 504 and 505 models.
The very last of these models included the 505 Series II GTD Turbo sedan with a 2.5-litre turbo four-cylinder diesel that delivered 70kW/206Nm as a manual only from 1987-89. It was a neat but rare package and is still sought after as Peugeot’s last rear-drive diesel. Previous 505 SRD Turbos offered only 59kW/184Nm from their 2.3-litre diesels.
Peugeot then swapped exclusively to front-drive models and offered the 405 SRDT from 1993-98 as both a sedan and wagon. Its turbo diesel four of 1.9-litre delivered 69kW/205Nm.
This series was replaced by the 406 series which offered a good choice of diesels from 1997 as the 406 STDT. Upgraded to 2.1-litre, this four-cylinder turbodiesel offered 82kW/251Nm until 1999 as a sedan and wagon. It was replaced by the 406 ST HDI, which shared the same body styles but offered a new 2.0-litre turbodiesel of 82kW/255Nm. This series ran from 1999-2004 until it was replaced by today’s 407 ST HDi models with an upgraded version delivering 100kW/320Nm from early 2005.
The small 306 models present a wide choice to budget conscious buyers. Launched first as the 306 XTDT five-door hatchback for 1996-97 with a 1.9-litre turbo four-cylinder with 69kW/205Nm, the diesel then came in the 306 XRDT/XTDT sedans only from 1999-2000 with 67kW/201Nm but were basically the same drivetrain. Both were replaced by the 306 XT HDi sedan upgraded to 2.0-litre and 67kW/211Nm for 2000-01. The new 307 2.0 HDi continued with similar outputs as a hatch or wagon from 2003-05.
Today’s 307 XSR HDI models were boosted to 80kW/240Nm from a 1.6-litre engine or 100kW/320Nm from the 2.0-litre in mid-2005 highlighting the impact of the latest technology. Most Peugeot diesels come as manuals only except for later 406 and 407 models.
Volkswagen
Volkswagen was a local pioneer in diesels offering affordable GLD diesel versions of its Golf and Passat models throughout the late 1970s into 1982. Unfortunately, the imported trim, paint and body fittings were no match for the long-life engines especially in rural areas where so many ended up. These models featured the diesel version of its 1.5-litre petrol engine with a relatively feeble 37kW/82Nm.
After a long absence, Volkswagen returned with a diesel version of the Series III Golf late in 1995. The Golf GL TDi was a marvel to drive with 66kW/202Nm from its 1.9-litre turbo four-cylinder diesel. It was sold here from 1995-97 but it did not continue into the Gen IV Golf.
The VW diesel returned as part of the current Gen V Golf range from August 2004 as the TDI with 103kW/320Nm from its 2.0-litre turbo diesel.
Volkswagen launched a Polo TDi diesel late in 2005 with 74kW/240Nm from its turbocharged 1.9-litre four-cylinder engine.