Audi's Turbocharged Direct Injection diesel engine with quattro all-wheel-drive(80251)Credit: Auto Express, , Audi, , Auto Express, , Audi, ,

The Evolution of Diesel Engine Technology

How diesel engines have come a long way to being the way they are today.

When people think of diesel engines, they usually imagine loud, rattling engines that emit massive quantities of thick black smoke that chokes the living daylights out of anyone around. However, those days are long gone, and diesel engine technology has come far beyond what anyone really expected. There are several engines in passenger cars today that defiantly stand up against larger petrol engines and come off victorious, and how. If you're a petrolhead and you're wondering what I'm on about, read on..


The Beginning

Ever since Mercedes introduced the very first diesel powered passenger car, the 260D, in 1936, they and many other manufacturers have been trying to put it to good use, by making it more compact and usable in more platforms. However, not much progress was possible then, and the diesel engine vastly remained in the confines of agricultural and construction equipment and other heavy applications.

Diesel engines are compression ignition engines. That is, they do not rely on spark plugs to ignite the charge of air and fuel, rather, they combust due to the heat produced under pressure created by compression. Thus, most engines were rather low speed.

Although diesel engines were used in some motor vehicles over time with technological advances in engine construction, they were still crudely designed and used indirect injection technology, which involved a pre-combustion chamber where ignition took place via compression, and the resultant mixture moved into the cylinder.

Direct injection followed, and was the first major step in the evolution process. Direct injection meant that the fuel would be directly injected into the cylinder along with the air. This would lead to more efficient combustion and result in more power being generated, and also better fuel economy. The engines also began to adopt four valves per cylinder, turbochargers and microprocessor controlled fuel management for better performance.



Even today, many cars or commercial vehicles still uses indirect injection. One of the key examples would be Peugeot's TUD5 diesel engine. This 1.5L four cylinder motor made 57 hp and 95 Nm of torque and powered several car models globally under license. In India, this engine powered the then present Peugeot 309, and also the Maruti Esteem and Zen diesel models, the Daewoo Cielo DLE and the Hyundai Accent DLS. Besides, Tata used their own 53hp and 85Nm 1.4L indirect injection 4-cylinder diesel motor in the car that would change the face of tourist taxis in the country - the Indica.

Incidentally, Royal Enfield also happened to introduce the world's first diesel motorcycle when they used a 325cc single cylinder diesel motor on a model they called the Taurus, which was based on the Bullet platform. Vibrations and unmanageable riding conditions saw it being put to almost every other imaginable use apart from riding by farmers and other locals.

A few years later, Mercedes and Hyundai were the two manufacturers than introduced into India the very first common-rail direct injection series of turbocharged diesel motors. Although the Mercedes C class with the CDI diesel engine was out of reach for many, the Hyundai Accent CRDi was not. Armed with a 1.5L three-cylinder engine making 81hp and 187Nm of torque at a rather low 2000 rpm, it revolutionized the idea of diesel cars and soon many wondered how this was possible.

Common-rail direct injection diesel engines were the first of the truly modern diesel engines and provided fantastic fuel economy, as well as much better noise levels and elevated performance when compared to previous generation direct injection diesel engines. They used a reservoir, or rail, to store fuel before injection. This ensured that the fuel would always be injected at a precise high pressure. A pre-injection shot, or pilot injection, which drastically reduces the noise from engine clatter and also serves to pre-heat the cylinder before the main fuel injection is implemented. This leads to cleaner more efficient combustion. Although several manufacturers from across the world had their own versions of common-rail technology for their diesel engines, Peugeot, GM-FIAT, Mercedes and Hyundai can be considered some of the very first to bring it into production.

With the threat that exhaust emissions pose to the planet, several crackdowns would soon come into play and the emission standards kept getting more demanding, with the Euro III norms or Bharat Stage III for India, leading to the demise of indirect injection diesels altogether. After the advent of Bharat Stage IV, common rail diesel engines were starting to be the norm. However, not everyone jumped onto the common-rail band wagon, with some manufacturers like Tata and Mahindra still making do with direct injection diesels with turbochargers and exhaust filters in quite a few models.



Diesel engines today are high-tech wonders of economy as well as power and torque. With the advent of common-rail high pressure direct injection, automobile majors began refining and honing the engine design to make it even better. This resulted in several technological advances that can be summarized as follows:

Cast Iron + Aluminum Construction: Cast iron engines were rather heavy for their own good, and while they were needed to handle the intense pressure, designers realized that not every component had to be made using cast iron. Aluminum was adopted and the weight of diesel engines saw a major drop.

High Pressure Common Rail: The fuel reservoir that holds the fuel before it is injected is specifically made to handle immensely high pressure, 2000 bar to be precise. Although most manufacturers don't run their engines up to those pressures, staying around 1600-1800 bar still presents great results. The rail acts like a buffer for fuel and always delivers precise injections every cycle through some minute injectors that use multi-hole designs to ensure fine atomization of every drop that leads to complete combustion and little or no waste of fuel.

Piezo-electric Injectors: Piezo crystals have been proven to react to electricity in microseconds, and hence are being used in injectors to provide precise timing for injection. They also allow a more accurate pilot injection, and benefit the overall design much more than previous means.

Variable Turbine Geometry Turbochargers: A turbocharger is basically an exhaust gas driven compressor that forces more air in for combustion. Paired to an intercooler, this compressed air turns dense and leads to more air and less fuel per combustion cycle maximizing the output of the engine while minimizing fuel consumption. While the blades of the fan in the turbocharger can be fixed (called fixed geometry turbochargers), newer designs allow them to vary the blade angle to optimize air pressure at all engine speeds. This reduces what is known as turbolag, or the delay that you experience when you put your foot down on the accelerator and the feeling the actual thrust from the engine.

Audi's massive 6.0L V12 TDI diesel delivering 500 horsepowerCredit: , , AudiCredit: , , Audi


Modern diesel cars are now proving to be far superior to their larger engined petrol counterparts. As you can see above, Audi's huge and advanced 6.0L V12 common-rail direct injection diesel engine makes 500 hp and 1,000 Nm of torque, that's as much power as the previous 8.3L V10 petrol-engined Dodge Viper had, and a whole lot more torque - 240Nm to be precise. And it's not just about power, while many petrol V8 engines need to be measured in gallons per mile, or litres per kilometre, these large diesels remain on the better side of that ratio, and still measure in mpg or kpl, and not the other way around.

Here's another more close related example. BMW, today, have a pair of 3.0L turbocharged straight six petrol and diesel engines, featuring the very best in technology. Both use BMW's TwinPower turbo that uses twin-scroll turbocharging technology and direct injection, with double vanos variable valve and camshaft timing for the petrol, and the most modern high pressure common rail system for the diesel. Here are the numbers -

Petrol - 326 hp and 450 Nm

Diesel - 313 hp and 630 Nm.

Yes, both are 3.0 litres, both have 6 cylinders and both have turbochargers. The petrol has 4% more power than the diesel, but the diesel has 40% more torque than the petrol. (And a noteworthy 5% more than the larger 4.4L V8 turbo petrol 50i). And no, the petrol is definitely not as economical as the diesel.

Another benefit is in drivability. With the massive low down torque generated by diesel engines, with no small role of the turbo, they are quick off the line without revving up the engine unnecessarily. This leads to lower fuel consumption from the very outset. Even if you compare smaller cars, for instance, cars like the Suzuki Swift or FIAT Punto, powered by GM-DAT and FIAT's 1.3L four-cylinder MultiJet diesel; the engine may have 75 hp, but it makes 190 Nm of torque from just 1750 rpm. That's more torque than a standard 2.0L Mitsubishi Lancer! Besides, it can deliver fuel economy figures of over 20 kpl (over 47 mpg US / 56 mpg UK) on a regular basis.

In the US, a lot of large SUV and truck owners are switching to diesel power, because it endows better performance for those kinds of heavy duty vehicles, without the nagging hefty fuel bills that they're much larger gasoline engined counterparts come burdened with. Prime candidates for the switches are Hummer H1 and H2 models, Chevy Suburban and Silverado, Dodge RAMs and the Ford F-series pickup range. Many owners of these vehicles choose Cummins alternatives or head to places like Predator Customs for performance upgrades.


Exhaust Emissions

When it comes to emissions, it just keeps on getting better. Nowadays, most diesel powertrains come equipped with a Diesel Particulate Filter (DPF). This traps whatever soot particles there may be, and in tandem with the catalytic converter, lead to emissions that are next to nothing. In fact, Volvo had run a test using their D5 diesel engine. When the S80 debuted with the new 2.4L inline 5 twin-turbo motor, a pipe was connected to the exhaust tail pipe and after a few minutes to allow the engine to warm up and the emission system to arrive at an optimal temperature, the pipe was put into the cabin through the side window. At that point, the air coming out of the exhaust pipe was cleaner than the air outside! This example goes to show how much diesel engines have evolved and the whole new avenues they open to powering passenger cars.


Summing Up

Of course, those who drive the two will always tell you that the bottom end grunt of a diesel out does that of a petrol motor by far. However, the way a petrol motor builds the revs and races towards the redline, which is always higher, makes it seem more fun to drive competitively for many. But then that beckons the memory of Audi and Peugeot diesel race cars dominating the legendary 24 hours Le Mans endurance race for several years on a trot. Then again, with the advent of turbocharging on almost every engine sold today, the gap between the two is closing, but greater economy and lower emissions continue to be the ultimate target for all manufacturers. In the end, we must reduce our dependence on conventional fossil fuels altogether. See my related article Economy Class Hatchbacks for more about that aspect. The race for the fuel of the future is on!