A gearhead’s love for cars is a highly subjective matter. Most like power, speed and noise. But on a deeper level, when spirituality and engineering marvel meet, a fascination emerges with how mechanical things work, and crucially for tinkerers – how they can be enhanced to evolve.
Back then, it’d be hard to classify the German Rudolf Diesel as a typical gearhead. The first cars – or horseless carriages – were only starting to see the light when he invented the eponymous fuel in 1897; and what’s more is that he sought an alternative to steam engines rather than gasoline ones.
Subsequently, in the early 20th century, the paths of gas and diesel initially diverged as the former was more readily produced, and passenger cars required light and compact sources of propulsion. Instead, diesel technology was implemented for the larger, heavier engines found in public transport such as ships and trains.
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Mercedes-Benz
While its roots trace back to the Benz Patent Motorwagen in 1895, Mercedes-Benz was officially founded in June 1926 when Daimler Motoren Gesellschaft and Benz & Cie. (the world's oldest car company) merged to create Daimler-Benz. This German manufacturer of premium cars is historically known for its motorsport pedigree in early Grand Prix racing, its off-road prowess in the creation of military vehicles like the Genadewagen (now G-Class), and its focus on utmost luxury and technological innovation in models like the S-Class and SL. Today, it is one of the world's largest automakers.
Only in the 1930s did automakers give diesel a second look; with 1936’s Mercedes-Benz 260D being the first-ever diesel passenger car. Yet, because of persisting low oil prices in the post-war period, no-one knew or cared about emissions. Amidst the great democratization of personal transport, diesel cars were forever shaded by their sexier gasoline counterparts – that sounded better, smelt nicer and drove more smoothly: an easy sell in the days of rampant consumerism and good times-giddiness of the Baby Boomers.
In 1957, Mercedes-Benz introduced the Mercedes-Benz 300D; a first not because it was diesel-powered as such, but because it was one of the first of its kind to implement forced induction, as well as proving that even high-end motoring could be enjoyed with diesel power.
Early icons like the Peugeot 504D and, in 1976, the Volkswagen Golf Diesel presented the technology as viable and affordable in the European market. Often derided for being slow and sooty, these typified all the bad traits of diesel. Yet in truth, they were the forebears of today’s 400 hp monsters – the world just didn’t know it yet.
How Diesel Engines Work
BMW
Although both operate on a four-stroke combustion cycle, unlike gasoline engines that rely on a spark to ignite the air/fuel mixture, in a diesel engine the fuel is lit through the compression of air in the cylinder. That’s because air heats up as it’s compressed; and in a diesel engine, fuel is injected much later just as the piston reaches the top during the compression stroke, thereby creating a longer-lasting downward power stroke.
The philosophy behind diesels is that higher compression allows more power and higher efficiency. Which is why they run higher compression ratios (18:1 up to 70:1) than gas engines, which in turn squeeze fuel and air into a mixture at a ratio of about 8:1 to 12:1.
The marriage of a more efficient combustion process to an energy-denser fuel, logically points to better fuel consumption figures; fewer stops at the pumps and a happier wallet.
Yet you may be equally pleasantly surprised when you pull up at the drag strip.
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Generational advances over the decades have seen to it that modern diesel engines are ultra-clean and where required – optimized for high performance.
The addition of smarter engine control in diesels – particularly from the 1990s onward; higher combustion efficiency attained by common rail, high-pressure, direct injection and variable-geometry turbocharging; as well as cleaner emissions management through particulate filters – have on paper made them almost indistinguishable from gasoline engines.
As a rule, when comparing same-size gas and diesel engines, a gas engine will have more horsepower but the diesel more torque and across a broader range of engine speeds, which translates to greater drivability.
Diesel fuel also has a higher energy density than gasoline – an average of 139 000 British Thermal Units (BTUs) against 124 000 BTUs per gallon.
We already know that a diesel engine’s higher compression ratio enables more torque to be produced. At the same time, as torque equals force multiplied by distance, a diesel engine’s longer piston stroke also contributes to greater torque production. And as they’re designed for turbocharging from the ground up, they can (and do) run higher boost.
A diesel engine’s longer stroke necessitates slower piston speeds, which, combined with more torque on tap, also means more of that is available from a lower rev range, which provides immediacy when accelerating off the line. Modern management systems coupled with offset-sized multi-turbo setups (BMW’s first M50d engine was initially fitted with four turbos, then three, as a testimony to tech evolution in its successor) are tuned and managed to deliver peak torque from table-top flat torque curves almost from idle speeds – to maximize drivability, towing or overtaking ability. In a way that a gasoline engine is only able to deliver from the mid-point to the upper end of its rev range.
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By the end of the 20th century, diesel-powered passenger cars were no longer a rarity, and enjoyed particularly in Europe for their longer range ability and economy in countries with high fuel prices.
But one frontier to prove diesel’s parity – perhaps superiority – with gasoline: the racetrack. Although in theory energy’s higher density would allow a car greater range (particularly useful in endurance racing), the thinking for a long time was that the extra weight of a diesel engine negates its benefit in a sport where mass and balance trumps all; and the low red line (typically 4500 r/min) is anathema to the high-revving nature of racing engines.
BMW's victory in a diesel car in the 1998 24-hour race at the Nürburgring ticked the final box. Incidentally, the company inadvertently missed a trick to cash in on its breakthrough success by focusing on winning the Le Mans the next year (with a gasoline car) and shifting their priorities towards (an unsatisfactory spell in) F1 from 2000 to 2008.
That left the door open for Audi, to not only introduce diesel technology to prototype racing in 2006 at the world’s most famous endurance race (and win on their debut), but also create awareness around the advantages and cleanness of diesel through its TDI marketing, particularly North America, which up to then was an untapped market – through participation in the American Le Mans series during the same period.
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At the zenith of its diesel drive, Audi even went as far as building a V12 engine for the Audi Q7 - the only of its type ever.
Between Audi and Peugeot (joining them at Le Mans between 2007 and 2011), a diesel engine won each running of the race between 2006 and 2014. Stricter performance balancing ended Audi’s dominance at Le Mans in 2015 and 2016; and the fallout of the Dieselgate scandal left parent company Volkswagen with no choice - politically, and soon, financially, too, because of the impending fines - but to terminate Audi’s diesel racing program from 2017 and beyond.
It was, of course, a corporate red-face moment of the highest order, but as the dust settled over time, Dieselgate proved a mere speedbump in certain countries. It killed diesel in the USA and Europe, but the fuel is still alive and kicking in other parts of the world. In the USA, diesel is mainly used in trucks to provide extreme torque output. But, did you know, in other parts of the world, the new Land Cruiser is available with a turbocharged diesel?
Source: The Evolution of Diesel Engines in Performance Cars
