how far can the diesel? The new super-diesel BMW is already official, with six in-line cylinders, four turbos, and 400 CV of power, to bring the benefits of the diesel BMW to a new level. These days I appeared at the BMW 750d xDrive 2016, the first model that will use this new engine with several assumptions: provide power, good performance and response, consumption of content, and autonomy is very high. But what are the secrets of the new super-diesel? why is it better to use four turbos, three and two?
why four turbos?
If anything, we are insisting, since BMW presented its diesel with three turbos, and since we had knowledge of this new diesel four turbos, it is precisely in that the increase in the number of turbos is not due solely to the improvement of the specific power of the diesel engine inline six-cylinder and 3.0-liter displacement of BMW.
The problem of turbo is that its operation depends on the pressure of the exhaust gases and, as a consequence, of the load on the engine. An engine can get excellent performance with a turbine of large size, that is able to introduce in the cylinder a greater volume of air, and at higher pressure. The big problem we found in that to spin a turbine of larger size also requires a higher pressure in the exhaust gas and the time delay, the time in which this turbine reaches its maximum regime of rotation, increases.
How can we solve the problem? Among the existing solutions, we find the electric compressors, turbines with variable turbine geometry, and use multiple turbos, with different stages, for each work at the right time. This diesel from BMW combines precisely these two solutions, of variable geometry and that of the supercharger for turbos in various stages.
How do you work the four turbos of the new diesel BMW?
We agree, therefore, that we want at least a turbo that turn fast, and without the need of a high-pressure exhaust gas, and a turbo that inexorably will take more time to spin up, and will require that the exhaust gases reach a pressure high, but thanks to which we will cover the middle and upper part of the rev counter. That is the basis of the technology of the new diesel BMW, and the reason for which they have opted for nothing more and nothing less than four turbos. A system of overfeeding in several stages, which in this case works with four turbos perfectly synchronized.
In normal conditions, we find two turbos of low pressure, in permanent operation. This is the great novelty of this engine compared to the diesel of the three turbos that BMW marketed to date. Before we met with a turbine of large size, working in the medium regime, which has now been replaced by two turbos, more compact, and with a more rapid response, that make this diesel offers a delivery more convincing to a regime of revolutions lower.
low regime, we also find one of the turbos high pressure, and turbine, low-inertia, permanent operation. This turbo is able to reach its maximum regime of rotation very fast, thanks to its turbine low inertia, and the variable geometry of the turbine. In fact, if you step on the accelerator to a regime of revolutions too low, it will close the valve of the turbos from low pressure to all the pressure of the exhaust gas is directed to this turbo, high-pressure, to get an immediate response.
To a regime most high, and from 2,500 rpm, comes into operation on the second turbo high pressure, and variable geometry, which helps to cover the delivery, we will require the regime medium and high.
In the video above, we see the operation of the diesel engine’s three turbochargers that until now had been used BMW. Engine will be replaced by the new diesel three turbos.
why is it better to use four turbos, three turbos, and two turbos?
Employ a system of two turbos sequential already provides a great advantage, as we saw above. The great advantage is that the first turbo, low-inertia, is capable of spinning very fast, until the pressure of the exhaust gases is sufficient to spin a turbine of large size of a second turbo to cover the supercharger regimes, medium and high.
With a third turbo, BMW managed to improve the response at low, and delivery in high, with a turbo of low inertia, working at low speed, a turbo, big size, working medium regime, and a second turbo low inertia working regimes higher.
The great advantage of a four-turbo is in having replaced the turbo of great size that worked half speed for two turbos that are smaller, who are now working throughout the revs, except if you accelerate to the bottom of a regime of low revolutions, a situation in which a valve closes the access of the exhaust gases to direct all the pressure to be a turbo low-inertia, variable geometry, and get an immediate response.
what figures are we talking about?
The BMW Series 750d 2016, the first model that will use this engine, accelerates from 0 to 100 km/h in 4.6 seconds. That is to say, it is only two-tenths slower than the BMW 750i xDrive V8-gasoline and 450 HP of power.
What’s most impressive, without a doubt, is that a regime in which any diesel is virtually dead, this engine gets a delivery very convincing. just 1,000 rpm, the diesel turbos from BMW delivery 450 Nm, and between 2,000 and 3,000 rpm you have already reached the climax, to 760 Nm. This engine also delivers 400 HP at 4400 rpm. An impressive figure.
Detail of the new engine of eight cylinders, two turbos, and an electric compressor, the Audi SQ7.
what Is the best solution, or the only one?
, much less the BMW is the only solution available, nor, necessarily, the best. To get an immediate response, Audi has already presented the first engine on the market with an electric compressor. What brings this technology is precisely a immediate response, with a system of compression of gases based on an electric motor, and as a consequence independent of the pressure of the exhaust gases, or engine load. See Audi SQ7 TDI 2016. The problem we found that this system requires to solve some technical challenges, such as to have an electrical system adequate to spin the turbine of the compressor at 70,000 rpm (Audi has opted for an installation of 48 volts) and to ensure its durability and reliability, which in these terms of performance nor is anything but simple.
we have Also seen how Volvo used an interesting solution, combining turbocharger turbine fixed and variable geometry, with reciprocating displacement compressors based on compressed air systems. The goal remains the same, deliver more power, but above all, get a delivery more high-to-low regime and an immediate response.
More information in “war of The systems Turbo: 5 technologies that you’ll be able to find out very soon bajot or hood”.
What solutions have been employed to improve efficiency and emissions for this diesel turbos?
In this new diesel engine four turbos we find other differences with respect to the diesel three turbos replaces it. It has installed a new system of recirculation of gases to the turbos, both at the stage of low pressure, as in the stage of high pressure. The goal of this system is to reduce levels of NOx that are emitted in the combustion, especially to regimes of high. This has required, among other things, the employment of a cooling system for the turbo separate from the employee in the engine, with a system of exchangers of heat.
This engine is, of course, with the catalyst of NOx, and a particle filter, but also with a system of reduction of NOx based on AdBlue.