Fleet News

Mercedes-Benz Diesotto



Right, this is something of a ‘Pay attention 007’ moment.

For one day in the middling future, you may have to explain to your driver why the car they have chosen to drive effectively has three motors that run like both a diesel and a petrol, and has some electric thrown in too.

Mercedes-Benz is developing the Diesotto petrol engine, first seen on its striking, fish-inspired F700 research vehicle.
Now it is powering an S-Class and feels altogether more real-world, and less Stingray.

Diesotto is a technology that uses spark ignition when the engine is cold, compression ignition as in a diesel for most middling running, back to a spark for heavy acceleration, with a hybrid electric motor smoothing out the transition between the two and adding a bit of muscle when accelerating.

It sounds clever, if a little convoluted, but when you consider that the S-Class I was driving has a 1.8-litre four-
cylinder engine, manages more than 50mpg and has CO2 emissions of about 130g/km, yet with 260bhp and a 0-62mph time of around seven seconds has performance equal to that of a current S350 petrol V6, you can see where this is going.

Dr Thomas Weber, board member of DaimlerChrysler AG responsible for group research and Mercedes-Benz Car Group development, says the goal is to make a petrol engine as economical as a diesel.

In order to do this, first the more efficient 1.8-litre unit is introduced. A two-stage turbocharger system, one big, one small, is introduced to add more power. 

Then, assistance from the electric motor of the hybrid module in the form of about 20bhp is added to the mix, with regenerative braking and stop-start technology.

Direct injection is hugely important, as it allows the engine to insert the perfect amount of fuel into the chamber for both spark and compression ignition at varying times, while variable valve lift adds greater flexibility. Exacting cylinder temperature and pressure control ignites the fuel under compression.

How hard is all of this to do? Well, extremely, as somehow you have to balance the three different modes of power in all conditions smoothly, efficiently and perfectly, like rubbing you stomach, patting your head, jumping from leg to leg and doing complex algorithms, all at 4,000 revolutions per minute.


Behind the wheel

This is an S350, to all intents and purposes. The only way you may get an inkling that something is afoot is when on the compression charge cycle, especially at lower speeds, there is a slight diesel-style chug. 

And the four-cylinder engine is never able to give you a sonorous noise as an engine with more cylinders does.

However, the engineers say they haven’t gone anywhere near to tuning the exhaust yet, and are confident they can smooth it out noticeably. Other than that, this Diesotto system is a marvel. 

Unfortunately you may have to wait half-a-dozen years to see the full show on the road, although elements of its technology will start to seep into Mercedes-Benz models over the next few years.

CO2 emissions and fuel consumption data correct at time of writing. The latest figures are available in the Fleet News fuel cost calculator and the company car tax calculator.

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