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The S250 CDI is the first 4 cylinder S class in a long time

By Rigval on 05 Oct 2010

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In compliance with the new Euro 5 emissions regulation Mercedes-Benz have announced a total of four new diesel and petrol engines for the Mercedes Benz S class. Out of the four, three are usual type of engines you see in a Mercedes S class – a 3.5liter diesel, 3.5liter petrol and a 5.0liter petrol with stop/start and all the BlueEfficiency and BlueTec Mercedes Benz eco-friendly mumbo jumbo but one is totally unique. A 2.2liter 4 cylinder direct injection turbo diesel engine in the S 250 CDI BlueEFFICIENCY . This engine is something you would find in the smaller E class or a mid range sedan, but not in the class topping S class. It is the first S class with a 4 cylinder engine in a long, long time.

I suppose the car industry is getting very environmentally conscious. Volvo has a small 1.6liter petrol engine powering their S80. Fiat has decided to put in a small 2 cylinder engine in their 500. Volkswagen has their 3 cylinder diesel engines in their Polo. It is an era of downsizing, and even the S class isn't immune to all of this.

But what an engine it is. I mean it has to be. Think about it. A 2.2liter 4 cylinder engine is able to pull a car the size and more importantly the weight of a S class. S classes are never lightweight cars. Even the lightest would weigh in excess of 1,750kg with all of the goodies and optional accessories their owners would spec their cars with. This is quite true as the S class has power this and power that. Everything needs a motor and electronics to work and these add weight.

So how does Mercedes Benz make a large car and a small engine work? The S 250 CDI BlueEFFICIENCY is equipped with a twin turbocharged 4 cylinder diesel and the latest-generation common-rail direct injection. The compact module for the two-stage turbocharger consists of a small high-pressure (HP) turbocharger and a large low-pressure (LP) turbocharger. The two compressors are likewise connected in series, and are in addition connected to a bypass duct. The combustion air from the air filter first flows through the low-pressure compressor, where it is compressed as a function of the LP turbine's output. This pre-compressed air then passes into the high-pressure compressor, which is coupled to the HP turbine, where it undergoes further compression. The result is a genuine two-stage turbocharging process.

The major advantage of this demand-related control of the combustion air supply using two turbochargers is improved cylinder charging, no lag and high torque even at low engine speeds. Fuel consumption is also reduced. The turbo setup above and the fuel injector system's twin high rail pressure of 2000 bar enables output and torque figures at the level of a 6 cylinder diesel to be achieved. It has an output of 204 hp, an amazing 500Nm of torque (for a 2.2liter production ready engine) and achieves a fuel consumption of just 5.7 litres per 100 kilometres (NEDC), corresponding to CO2 emissions of 149gm/km.

The S 250 could be the world's most fuel-efficient luxury saloon. That 500Nm basically guarantees decent performance for the heavy saloon.0-100km/h is as good as the S350 hybrid – 8.2seconds and also a top speed of 240km/h. Not S65 AMG fast, but totally able to keep up with the cut and thrust of modern day traffic. And technically that is all the performance you need when you're the owner sitting at the rear seat reading the Business Times while your driver is driving it. .

This 4 cylinder engine is also the first four-cylinder engine in the more than 60-year history. It is also the first car in the luxury segment to run on under 6 litres of fuel per 100 km (Lexus LS600h, eat your heart out), and the first vehicle in its class to attain CO2 emissions below the 150 gm/km mark. The S 250 uses an extensively revised, high-efficiency 7G-TRONIC PLUS 7-speed automatic transmission and the ECO start/stop function. This package is supplemented with other specific BlueEFFICIENCY measures such as the load-controlled oil pump and tyres with lower rolling resistance.

The question now is how refined can a 4 cylinder be? Firstly, a diesel is supposedly less refined than a petrol engine. These days this isn't as bad anymore due to direct injection and also turbo chargine. But a 4 cylinder is dynamically unbalanced compared to a V6, in line 6 or a V8. So to ensure S class levels of comfort, dynamically controlled engine mounts feature as an innovation in the S 250 . The rigidity and damping of these mounts alter according to engine speed, load and vehicle speed, thus preventing the transmission of unwanted vibrations and noise from the engine to the body. Porsche also uses this type of altering engine mounts for their latest 911 GT3 RS. Something like the magnetic shocks used in most performance cars, which alter its firmness upon demand.

I think this is an amazing piece of engineering by Mercedes Benz. What they are trying to do is to demonstrate to people that you actually can drive a luxury car with a clear conscience as it does use less of the oily stuff compared to any other large luxo-barge. It actually sips diesel like a mid sized family car and that people, is truly amazing. 500Nm worth of amazing.

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Written by Rigval
Born in 1972. Married with a kid. Loves B-road drives and have driven cars from the 1950s to date.

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