Quote:
Originally Posted by bradleyland
I'm not trying to be hard on you, but you're out of your depth again. You've got part of the story right, but you've tripped up on the details. It's true that diesel EGTs are typically lower than gasoline powered engines, but their exhaust gas volumes are much higher.
A diesel engine has no throttle plate. RPM is governed by the amount of fuel injected in to the cylinder. This is possible because you needn't maintain the strict 15:1 (or very close) air-fuel ration in a diesel. The full volume of air can flow through a diesel engine at all times, which is why their exhaust gas volumes are higher.
Because of this, you cannot simply swap a turbo diesel setup to a gasoline powered engine and expect things to go swimmingly. The turbine and compressor maps will very likely be mismatched to the gasoline engine.
I could be wrong. I mean, we're all guessing here, right? Maybe BMW goes all retro on us and drops a sequential setup on the rumored S20 engine, but I think that's unlikely, if only for cost reasons. The benefits just aren't great enough when compared with something like a twin scroll setup.
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I'm enjoying how I'm being paternalized here... all of a sudden the forum is full of mechanical engineers and PhD academics.
Look, it's too much talk and very little relevant information about a very simple matter.
I'm not guessing, I'm presenting a possible solution for throttle response quality hyper concerned people like yourself, and you are indeed wrong except when you've mentioned that the main issue about a twin-turbocharged solution for a mass-market car is its cost viability, that's the reason why they came up with the twin-scroll solution, one is cheaper than two.
You see, long before being used on diesel car's engines the twin-turbocharged solution was used on no cost-object super sport cars such as the Mazda RX-7, the Toyota Supra and the Porsche 959 which were a way ahead of their time.
In fact, "the use of sequential twin turbochargers (two small KKK turbochargers) rather than the more usual identical turbochargers for each of the two cylinder banks allowed a smoother and seamless delivery of power across the engine RPM band, in contrast to the abrupt “on-off” power characteristic that distinguished the 959 engine from Porsche’s other turbocharged engines (930 and also 964 turbo) of the era. Sequential turbo charging was controlled as follows: below 4,000 rpm, as exhaust gas was not enough for driving both turbines efficiently; all the exhaust gas was fed to a single (small) turbo. This made the turbo operating earlier and therewith allowed the engine to build up boost earlier compared to using a conventional turbocharger setup. Between 4,000 and 4,200 rpm, the second turbo started “pre-spinning” (preparing for engaging soon at higher revs). Above 4,200 rpm, the two turbos operated simultaneously to provide full boost".
Also, the computer-controlled waste gates now used on the S55 engine of the M3/M4 were, naturally, already used on the 959's engine twenty five years ago.
What you find on the BMW's twin-turbocharger inline-4 diesel engines is exactly the same principle, except for the size of the second turbocharger which is bigger than the first one in order to achieve higher boost while the turbo lag is reduced even further.
So, if low inertia, exhaust manifold integrated small twin-scroll turbines on the modern turbocharged inline-4s are not enough for you as far as throttle response quality goes the above solution is what I would recommend to BMW for the S20 engine.
Why is the inline-4 engine so important then?! Because with turbocharged inline 6 cyl. modern BMWs not only aren't as fuel-efficient as they could be with 4-cyl. engines but also they are too nose heavy which translates into poor turn-in characteristics, and ultimately less agility or sporty character, and less than ideal braking performance.
Bottom line, the inline-6 cyl. engines are today a marketing thing for BMW.