Like putting peanut butter on pizza, sometimes some ideas that seem promising on paper just don’t pan out in practice. Looking at the case of hybridized gas engines versus diesel ones, one makes obvious sense and the other… less so.

Gas engines tend to have more vertically shaped torque curves and when there’s a turbocharger fitted, the engine first needs to build revolutions to produce exhaust gasses to spool up the turbo before meaningful torque can be produced.

By contrast, compression ignition combined with the higher energy density of diesel fuel enables greater efficiency than in a gas-powered engine. Diesel engines also deliver more torque from the get-go and thereafter as a table-top-flat curve, greatly aiding a car’s driveability and overtaking ability at low and medium speeds.

So, by just comparing the torque traits of each engine type, it’s obvious that adding a hybrid system to a gas engine, whether turbocharged or naturally aspirated – as in the case of the Toyota Prius – it is better suited to supplanting those dreaded missing get-up-and-go in gas engines.

That’s not to say hybrids can’t be co-opted with diesel power. Diesel-electric trains are often cited as counter-cases in point. However, in such arrangements, the diesel engine fulfills no propulsive purpose other than to operate as a generator for the electric motor.

Neither are mild-hybrid installations – with tiny batteries acting as starter-generator-motors and providing minor levels of off-the-line encouragement – uncommon.

And, lest we forget, Audi racked up three straight Le Mans victories between 2012-2014 with a diesel hybrid engine. Had they not withdrawn at the end of 2016, who knows how many others would have followed?

While there are examples of diesel hybrid models, this piece will explore why this configuration isn’t more readily available.

Why Diesel Hybrids Aren’t A Good Fit – Mechanically

Efficiency. Gas-running engines are less efficient than diesel ones. In a typical application, gas engines burn about 25% to 30% of the fuel’s available energy – the rest is lost to wasted heat and noise. Diesels, on the other hand, burn up to 35% of the fuel’s available energy. Because the latter sports an inherently higher energy efficiency out of the box, the gains made in fuel economy by adding hybridization won’t be as significant.

Cost. Diesel engines are up to a third more expensive to build than their gas counterparts. This is because of the use of more durable components needed to withstand the higher compression ratio and higher cylinder temperatures required to ignite the diesel (no spark plugs are used). Furthermore, high-pressure common-rail injection systems in diesel engines are more complex than those found in gas engines, as are emission control systems such as particulate filters (DPFs) or catalytic filters that are essential for creating regulatory-level tailpipe gasses.

As a consequence, for diesel cars to merely meet emissions regulations charges up production costs from the outset (can you say Dieselgate?), and then you haven’t even added any hybrid components yet.

Complexity. A hybrid system comprises one or more electric motors, batteries and a control unit, which all have to be integrated into a vehicle’s design. As each car’s manufacturing profitability is ultimately determined by a calculator, the addition of this extra hardware to an engine that’s already more expensive to build makes it less affordable and even out of touch with a segment’s needs. Diesel engines are also heavier than their gas counterparts – as are electric motors and batteries. Pairing them will spike the scale, which in turn limits a vehicle’s ultimate efficiency.

Operation. The game-changer that’s made diesel engines more socially acceptable has been the ability to neutralize their emissions. Diesel engines operate optimally with constant heat, and, equally crucial for environmental reasons – so do the DPFs in their exhaust systems. The characteristic stop/starting that gas hybrids call upon to reduce their emissions would limit the ability to generate sufficient exhaust heat necessary for the DPF to perform at its peak.

Torque delivery. Electric motors work best at low speeds and are ideal for uhm, plugging the torque gap that gas engines suffer from in the lower part of the rev range. As diesel engines by default don’t suffer from a lack of bottom-end torque, adding an electric motor merely replicating what’s already there in abundance, doesn’t really make sense.

Why Hybrid Diesels Aren’t A Good Fit – Existentially

That’s not to say the world hasn’t seen mass-produced diesel hybrids yet: the Mercedes-Benz E300 BlueTec, Peugeot 3008 Hybrid4 and Citroën DS5 Hybrid4 all featured the technology, but failed to wow customers.

It must have been heart-breaking to see diesel fall from grace, so publicly and so suddenly, after all the billions that the Volkswagen group (which owns fellow diesel burners in Audi, Porsche and others) pumped into making cleaner diesel cars – by hook or crook – towards popularizing the concept. And proving its worth through projects like its monstrous Le Mans effort.

Post-2015, because of the diesel scandal, such-powered cars have become almost unsellable everywhere in the world, and particularly in the US. The fuel’s appeal all but disappeared in the passenger car market, though they survived in the pickup and heavy truck segment.

You could argue that Dieselgate was an evolutionary invention that forever changed the direction of internal combustion engines. While not entirely wiping out the segment, perhaps as a knee-jerk reaction, the immediate fallout thereof notably accelerated the push for electrification within the Volkswagen group alongside stricter emission regulations being promulgated.

Why You Won’t See A Diesel Hybrid Anytime Soon

With diesel rendered a swearword in the late 2010s – albeit a proven and mature propulsion alternative, by convenience and necessity the automotive industry’s focus simply shifted to gas hybrids in the interim and electrification as a long-term goal. In the early-2020s, as future dates were published that would ban the sales of anti-internal combustion cars in Europe, there was even less reason for car companies to pursue any further development associated with diesel technology. As demand declined but stabilized, diesels were reduced as bit-part players in car companies’ product portfolios: still present but no longer shouted about.

An industry-direction-altering scandal or not, in truth, the concept of an über-efficient super-engine diesel hybrid was stillborn, as it faced threats within the company quarters – at the costing department as well as the factory floor. Today, outside – evolution and legislation.

Which only underlines the importance of never letting a good crisis go to waste.

Sources: Greencarreports.com, Hotcars.com, Mansfield Service Partners, Overdrive.in