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First drive: Merc's high-tech DiesOtto engine

2008-06-26 09:22
Mercedes, Diesotto, S-Class, F700

Egmont Sippel

So, Mercedes-Benz is for old toppies, you say.


But only up to a point, nowadays.

Take the CLK63 Black, for instance, with its totally redesigned front end in terms of steering and suspension. Boy, it’s responsive and agile – precisely the type of dynamic support AMG’s massive performance has been waiting for.

Benz utilised the same front end to gate-crash the compact power sedan party with their brilliant new C63 AMG, too.

Which just means that Stuttgart’s model line-up is now fronted by a double-barreled bazooka assault as compelling as Keeley Hazell’s. They’re unstoppable, those two 63s, because they handle so well.

And there’s nothing better than ample curves to satisfy the lusts of red-blooded young enthusiasts.

In mountain passes, of course.

Yet the selfsame C63 AMG, good feel and great handling notwithstanding, leaves Stuttgart open to another left field jab: 6.3 litres, non-believers say, is a lot just to keep up with 4.0 and 4.2 litre V8s from BMW and Audi.

For younger folk also

Ja, well, no fine.

It’s 6.2 litres, to begin with – but that’s besides the point.

What matters is that the Benz does top power and torque tables and that price is pretty competitive, too.

Forget about power per litre, then. Consider bang for your buck in a chassis that, at long last, can handle, and it’s clear that the modern three-pointed star appeals to more than just old fogeys.

Yet, consider the following if you’re still not convinced: Stuttgart’s new small-capacity prototype petrol engine makes mince meat of your better-than-average turbo-charged pocket rocket power plant.

Yes, the DiesOtto – as this technological masterpiece is called – delivers a full 175 kW from only 1.8 liters and four cylinders.

That’s impressive, even with the support of two turbochargers.

But here’s an even more astounding figure: the DiesOtto delivers not the measly 300 or 320 Nm of your certifiably wild 2.0- or 2.5-litre turbo-charged hot shots.

Nope. The DiesOtto’s knock-out punch registers a full 400 Nm on the Richter Scale of Twist.

That’s before you’ve added the muscle from an electric motor on the crankshaft.

Whilst power and torque are hugely impressive, then, the DiesOtto is indeed also a mild hybrid.

And more: it’s revolutionary.

Astounding DiesOtto statistics

We’ll get to the mechanics in a minute.

But here are the results: consumption of 5.3 litres per 100 km (and we’re talking petrol); CO2 emissions of 127 g/km; negligible NOx pollutants; a 0-100 km/h sprint in 7.5 seconds; and a top speed limited to 200 km/h.

Not great, you’d say about the last two; Merc are still building cars for old toppies.

Think again. All of this is applicable to a vehicle exceeding 5 metres in length and pushing 1 800 kg. That’s a metre longer and half a ton heavier than your average little hot hatch pulled along by a bigger engine than the DiesOtto, whilst gulping twice as much juice.

Now, until alternative fuels are viable, we’ll have to depend on a clever use of existing technologies to lessen the burden on Mother Earth.

DiesOtto is an interim answer.

In fact, having had the exclusive privilege as a South African journalist to drive an S-Class mule equipped with this engine in Spain, I’d be tempted to say that the ground-breaking DiesOtto just might be the answer.

Merc was also kind enough to drive me around a race track in their fascinating new F700 concept chariot powered by the DiesOtto. Having been built on Merc’s Maybach platform, the F700 boasts a wheelbase like the Mulsanne straight. The car is furthermore distinguished by dramatic swoops borrowed from aquatic themes.

But that’s a story for another day.


Today’s focus is on the DiesOtto.

And let me tell you that this puny little 1.8-litre 4-cylinder uses a solid stream of thrust to simply sling big monsters like the S-Class out of corners, the turboes taking over seamlessly from the electric motor to power the car on to surprising speeds in a short distance.

Hybrid technology switches the DiesOtto off when the car is stopped, of course, to save even more fuel. Release the brake pedal and the mill springs to life again. Push on the accelerator and off you go without sacrificing any traditional S-Class traits such as power, performance, luxury and refinement.

Plus you clock up huge savings on fuel and emissions.


How is it all done, then?

The DiesOtto combines classic characteristics of an Otto cycle engine – refined, high-revving performance from a spark-ignited mill – with renowned advantages of Rudolph Diesel’s self-combusting engine – good efficiency and torque from meagre consumption, resulting (in the DiesOtto’s case) in low pollutant levels.

Does this mean that the DiesOtto runs on petrol and diesel?

Nope, it runs on petrol only, even though it utilises a spark-ignition mode and an auto-ignition mode.

At start-up and idle, for instance, the engine is simply too cold for auto-ignition.

And under hard acceleration, and serious driving at the top of the rev range when throttle openings are big, compression ratios are capped (to a figure below the threshold of auto-ignition) to prevent engine knocking and damage resulting from that.

Under steady normal driving conditions, however – when the engine is only under part load – it is entirely possible to initiate auto-ignition in a petrol mill without incurring engine knock.


The CAI (Controlled Auto-Ignition) phase of the DiesOtto engine would commonly be utilised at anything between 1 000 and 3 500 r/min when the car is travelling at, say, a fairly constant 60 km/h in city traffic or 120 km/h on the highway.

In other words, in driving modes often frequented by the average motorist.

The end result is big cuts in consumption and CO2 emissions as well as NOx emissions, the latter precisely because auto-ignition ain’t reliant on a spark followed by flame propagation, but rather on a quick and relatively cool burning process with dramatically less NOx emissions.

Consumption, on the other hand, is lowered via a very lean air/petrol mixture (lambda 1.5, or 22 parts air for every 1 part petrol).

This is made possible by a thorough – or homogenous – mix of air and petrol, resulting in easier ignition.

On the intake stroke, for instance, the intake valves open a bit later than normal to create low-pressure conditions in the combustion chamber, precisely to suck air in with a rush when those valves eventually open again.

In the mean time, a good bit of hot air from the previous exhaust stroke has been trapped in the selfsame cylinder. This is done by limiting the lift and opening period of exhaust valves through the use of smaller and lower secondary lobes on the exhaust cam (as in Honda’s V-TEC technology).

The trapped heat will be enough to vaporise the directly injected petrol, further to facilitate the formation of a homogenous air/fuel mix completely filling the combustion chamber.

So evenly will the mix be spread, in fact, that the vaporised fuel will in effect act as thousands of little ignition points, or “spark plugs”.

After all of this, only one more thing is needed to auto-ignite and that’s extra heat.

That heat won’t arrive via a spark, of course. Once the conditions for auto-combustion have been created (part load, even speeds, the right rev range, etc) the engine switches from spark to auto ignition in a single cycle, at which point the spark is stopped.

What to do, then?

Variable compression ratio

A-ha, a variable compression ratio could do the trick, not so?

Think of a bicycle pump. The harder you work it, the more heat is generated.

But how to vary this ratio which, in the DiesOtto, ranges from 7:1 to 14:1?

There are various patents out there, but Merc is unfortunately keeping mum on theirs.

Not to worry, though; the rest of the DiesOtto is complex enough in terms of so many advanced technologies and electronics. The engine will therefore take some time yet to reach the market; many reliability and durability tests must still be done, costs must be curtailed and a couple of ECUs must be integrated into a single unit.

And then?

Well, your C-Class Sports Coupe will sip less than 5 litres/100 kms and still be launched from 0-100 km/h in less than 5 seconds.

Which is not really old toppie territory, to say the least.

Egmont Sippel is Rapport’s Motoring Editor and SA Motoring Journalist of the Year 2007/08.


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