BMW M3 / M4 Engine (S55): Deep Dive and Interview

BMW M3 and BMW M4 Inside (part 3): The new engine.
Interview with M engineer Juergen Poggel, Head of Development M Engines.

So a new engine has been developed for the BMW M3 and BMW M4?

Jürgen Poggel: Yes. The previous engine was a naturally aspirated V8. For the new BMW M3 and BMW M4 [full technical info], we in the M engine development department in Munich’ Preussenstrasse have designed an M TwinPower Turbo inline six-cylinder design and readied it for series production.

What is special about this new engine?

In terms of functionality it is characterised by its turbo-typical constant high torque, which is available across a very wide band, from 1850 to 5500 rpm.

Its performance curve is outstanding: its maximum power is available over a wide plateau of 5500 to 7300 rpm – and its maximum engine speed is as high as 7600 revolutions per minute.

The engine represents the best of both worlds – turbo and high revs.

BMW S55 Engine
What are the benefits to the customer?

When the driver takes the engine up to 7300 rpm, full power is immediately available again after changing gear – precisely because the maximum power is available within an engine speed band of 1800 rpm. If the BMW M3 or BMW M4 is equipped with M double-clutch transmission, gear changing can be performed with no interruption to the driving power. The integral of power over engine speed is therefore considerably higher than it would be if the maximum power level were only available briefly as a peak value.

This results in a phenomenal impression of acceleration.

History of power (red), torque (dark blue) und CO2 emission (light blue) of BMW M3 automobiles.
So how is the fuel consumption of this car?

If you take it to the peak of its power, you will get about the same consumption levels as the previous engine. When it is running at partial load, help is provided by the high torque level, which means that consumption in normal road traffic is easily two or three litres less. The consumption spread has moved downwards overall while average consumption levels are correspondingly and discernibly lower.

What had to be done to achieve this result from a technical point of view?

We subjected the engine to a consistent process of dethrottling. The intercooler that sits visibly on the engine is positioned directly in the line between the compressor and the intake system. Exhaust runs are also as far as possible rectilinear and dethrottled. This enables a high and broad performance plateau with lower consumption levels.

A particularly important aspect of power delivery in an M engine is its response characteristics…

Everyone who has tried out this engine has confirmed to us that here too, we have got our sums right. Both Timo Glock and Bruno Spengler, who helped us out with the final setup runs on the north loop of the Nürburgring, were thrilled.

What is this due to?

Even the naturally aspirated full load level extends to as much as 270 Nm. The car will remain in this range when you want to keep up the power in the vehicle when driving through various kinds of bends. You will not feel any delayed response of the turbo in such cases, for the simple reason that the turbo charger has not actually come into play yet at these speeds.

When accelerating out of fast bends, the car will enter the turbo range, and all measures have been taken to ensure a fast response. We use two small turbo chargers with a very high efficiency factor and a low moment of inertia. Plus the dethrottling leads to very low levels of flow loss.

Turbine wheel.

This time there is no cross bank exhaust manifold as there was in M cars with the M TwinPower Turbo V8 engine?

Well, actually, from a functional point of view there is. We have created an optimum system for collecting the exhaust fumes from each set of three cylinders. The difference is that in this design the cylinders are in an in-line arrangement.

The response characteristics of a turbo charger is often assessed quite differently by different people. What could be the reasons for this compared with a naturally aspirated engine?

One difference is that with naturally aspirated engines, there is a stronger intonation of intermediate positions of the accelerator pedal. If you increase the acceleration slightly, the engine immediately sounds different. A naturally aspirated engine acknowledges the request for more power with a change in sound. But with a turbo engine, this is not resolved acoustically in such detail, because the noises on the intake and exhaust sides are heavily damped by the turbo. It is this lack of acoustic feedback that creates the subjective perception of the engine’s response.

The entire drive train and its control system also have a major effect on power delivery. It is quite possible that due to the good response and high torque of the M TwinPower Turbo engines – if it is switched on – the control system comes into action to prevent any breakaway in the tyre adhesion. In such cases, the DSC acts precisely, to keep you directly within the slip limit. This can be easily observed in serpentine bends – the DSC only releases the power again to a large extent once you have passed through the curve. The control systems work so well that you can hardly tell that it is this system that is currently determining the maximum possible acceleration level. If you switch off DSC and activate SPORT PLUS, you can feel the outstanding response very clearly, and, for instance, make good use of it on the racetrack to influence driving dynamics using the accelerator pedal.

There are three driving modes: EFFICIENT, SPORT and SPORT PLUS. In terms of engine activity, do these simply alter the accelerator curve?

No. With an M automobile, racetrack use is always an essential factor. Here it is necessary in all cases to be able to control very high power outputs with great precision using the accelerator pedal. For us, this means that we would not even consider a mode in which a small pedal movement would produce a great leap in power. In such a case it would not be possible to produce a measured power output. Which is why we prefer to create a largely linear torque curve controlled by the travel distance of the accelerator pedal.

So what are the differences between the EFFICIENT, SPORT and SPORT PLUS modes?

In the SPORT and SPORT PLUS modes, the waste gates remain closed under partial load and the mass flow is routed through the turbines, to ensure that the turbo chargers are already rotating at a higher base speed even in standby. Moreover, certain special engine control functions are brought into play that accelerate activation of the turbo chargers. This means that the car responds more closely to the accelerator in SPORT and SPORT PLUS modes.

Another important point is that we normally have to take the drive train into consideration when it comes to building up torque. As a rule, the engine is able to build up torque at a speed that is much faster than that with which the drive train can cope. You can imagine that the drive train then begins to behave like a torsion spring, which is first stretched out and then vibrates, resulting in juddering. To prevent such judders, it is necessary to dampen the engine’s build up of momentum to allow a harmonic progression of its response characteristics.

The rigid drive train of the new BMW M3 and BMW M4 is, however, optimised for a fast build-up of momentum, similar to a racing car, with a carbon fibre drive shaft, rear axle rigidly bolted to the body, suitably dimensioned drive shafts, etc. This means that the rigid drive train is also an important factor of the spontaneous entrance of the new BMW M3 and BMW M4. In EFFICIENT mode, everything is more levelled out, with a high orientation towards comfort. In stark contrast is the SPORT PLUS mode, which pays absolutely no attention to any comfort factors. In SPORT PLUS, all filter functions are deactivated and the engine responds directly to movements of the accelerator pedal – but this also requires someone behind the wheel who really knows what he is doing. In SPORT PLUS, everything is transmitted as sharply as a knife, in terms of what the engineering can do to facilitate fast lap times – whether in the engine, transmission, chassis or control systems.

Thank you very much Mr. Poggel – we look forward very much to seeing the new engine in the BMW M3 and BMW M4 for ourselves.

Source: M-Power