Thursday 21st July 2011

New 422hp AMG V8 Gets '4-Pot' Mode

Cylinder deactivation system promises fun and economy for SLK 55

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The 4 (or 8) cylinder SLK 55 AMG

PH got a privileged invite to Affalterbach yesterday, where head of AMG powertrain development Fritz Eichler pulled the wraps of the company's new M152 engine - a normally aspirated variant of the 5.5 litre M157 'biturbo'.

The engine is notable for its F1-derived cylinder deactivation system and 'stop-start', which means the new SLK 55 AMG (in which the engine will make its debut at the Frankfurt motorshow in September) should combine supercar performance with class-leading efficiency.

With all cylinders firing the M152 engine produces a heady 422hp at 6,800rpm and a whisker under 400lb ft at 4,500rpm. Yet coupled to the latest version of AMG's existing 7 speed torque converter auto (chosen because it works best to smooth out the transition between 4- and 8-cylinder operation), the SLK 55 is capable of up to 33mpg on the combined cycle. That's an increase in efficiency of 30 percent over its predecessor, while CO2 emissions have also been reduced to 199 grams per kilometre.

Fritz Eichler and the new M152

The four-pot mode only works in transmission mode 'C', and apparently the switch from 8 to 4 cylinders and vice-versa is seamless. If you want to accelerate hard from 4 cylinder 'eco' mode, the switch to 8 cylinders takes just 30 milliseconds, so it happens entirely imperceptibly while the gearbox kicks down. (They say.)

Cylinder shut-off works between 800rpm and 3600rpm, by cutting fuel to cylinders two, three, five and eight. The valves on the deactivated cylinders remain closed on the compression stroke, so there's a kind of spring rebound effect from the compressed fresh air within the cylinder which allows the load change losses to be reduced. (Unless Herr Eichler lost us on that point, which is entirely feasible...) A 'centrifugal force pendulum' has also been introduced into the transmission to eliminate nasty four cylinder vibrations.

In case you were wondering, the system can't be introduced on the M157 itself, (although the majority of its components are identical), because AMG hasn't yet worked out how to seamlessly integrate the turbos - although you can bet they're working on it.

Getting the lowdown on the new unit

You might also be wondering what a four cylinder AMG engine is going to sound like? Well, we can't answer that yet, but the company says it has been working hard to make sure it doesn't offend expectations. To that end the SLK 55 AMG will be the first AMG model to feature variable flaps in its exhaust system, specially tuned to eliminate some of the treble notes and amplify the bass during 4-cylinder operation. Under full load the flaps in the silencers are opened, so you can enjoy the traditional V8 thunder.

Speaking at the engine's unveiling, AMG chairman Ola Kallenius was keen to impress on us the importance of technology such as this to the AMG brand.

"We want to be the driving force in performance systems going forward, but when we do something new we have to be true to the AMG gene and at the same time we have to be efficiency leaders.

"We like to say this is our first '99 gram' CO2 car, but with a one in front," he quipped, "because with an efficiency gain as great as 30 percent it feels like a 99 gram car for the performance sector."

As long as it doesn't drive like one, we'll all be happy...

[Tyre Tread]

It's an invitation not an invite! If you journo's can't get it right how can we jump on the others?

[MG-FIDO]

"Cylinder shut-off works between 800rpm and 3600rpm, by cutting fuel to cylinders two, three, five and eight."

Surely such humble revs are never seen by the average PistonHeader!?


Whilst similar things have been done before, the idea of cylinder shut off has to be a good idea if it works as intended!

[Targarama]

Didn't Cadillac do this years ago on one of their guzzlers? Seems like a good idea to me. Cut the consumption when you're cruising along and not needing the power.

[Wadeski]

Cadillac tried it mechanically in the 80s (disaster) and more recently electronically (success).

[Tyre Tread]

Targarama said:

Didn't Cadillac do this years ago on one of their guzzlers? Seems like a good idea to me. Cut the consumption when you're cruising along and not needing the power.

I believe the Dodge Ram has the facility so the idea isn't new although I expect the execution is more refined.

[5potTurbo]

Chevrolet have been doing this for years.
My rental Suburban last year shut down to a V4 when "cruising" on freeways/interstates. Managed over 35mpg.

Oh, and yes, to post #1!

[alexpa]

The relatively simple GM small block V8's produce similar power/ torque/ mpg figures. Doubt g/co2 are similar though.

[AyBee]

Like this a lot All the gains attributed to the combined performance and efficiency with regards to tax and fuel consumption and yet if you drive it properly, you use all 8

[Ved]

Wonder how this will affect the carbon emissions though. If a 80k AMG gets 20mpg or 35mpg it probably doesn't make much odds to someone who can afford that. However, if the tree huggers were to test this in 4cyl mode and get the tax and emissions down then everyone wins... except the tree huggers

[PaulMoor]

"We like to say this is our first '99 gram' CO2 car, but with a one in front"

German and an engineer...

[Matt UK]

Like it a lot.

You get taxed in one mode and actually drive around in another. Neat idea

[kambites]

I suspect even the most aggressive of drivers will end up driving on 4-cylinders the majority of the time. I wonder what it does to the low-rev, low-throttle engine note?

[loveice]

So, this latest F1 related 5.5L V8 from AMG produces less horse power than a nearly 5-year old 4.0L V8 from BMW's M...

[The Wookie]

Article said:

'centrifugal force pendulum'

Did 'balancer shaft' sound a bit too boring?

[kambites]

loveice said:

So, this latest F1 related 5.5L V8 from AMG produces less horse power than a nearly 5-year old 4.0L V8 from BMW's M...

...3. BMW never put that engine in a sports car.

[astrsxi77]

It's a clever idea and, as per comments above, it has been done before, most certainly by GM (Northstar V8?).

But I have always wondered whether these systems result in rather uneven engine wear, long term. Surely, if it's always the same 4 cylinders that "shut off", and depending on how the car is driven over a given period of time, the 4 remaining pots and associated values doing the majority of the work will experience accelerated degradation and wear?

[kambites]

Why would the wear on the active ones be higher? They aren't moving any more than they would if the other four cylinders were active. It might mean that the deactivated one wear less than they otherwise would, but I don't see how that can be a bad thing.

[Greza]

Old technology according to this

http://en.wikipedia.org/wiki/Variable_displacement

[Steve Gunnis]

Does this mean that cylinders 1, 4, 6 & 7 will wear much quicker than the other four? Is this a good idea? Wouldn't it be better (and no more complicated) to alternate between the deactivated cylinders in four cylinder mode?

I wonder what output the four cylinder has? With the V8 only making 422bhp (76bhp/litre is pretty terrible specific output these days), a rev limit of 3800rpm and the internal losses associated with four working pistons spinning a V8 crank and four other 'dead weight' pistons, not a lot. It's going to feel like driving a 1.4 Polo towing a caravan.

[thinfourth2]

So how does it work does the pivot for the valve go up and down instead of the valve?

kambites said:

Why would the wear on the active ones be higher? They aren't moving any more than they would if the other four cylinders were active. It might mean that the deactivated one wear less than they otherwise would, but I don't see how that can be a bad thing.

Cue a long argument about gas pressure behind piston ring and bore polishing