Quote:
Originally Posted by Boss330
Please explain the in-efficiencies that is connected with the tri-turbo system in the M550d engine. It seems to me that the advantages must have been greater than the disadvantages since BMW has chosen this concept?
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If your goal is to reduce boost threshold to very low revs, without compromising top end power, then yes a tri turbo like M550d is a valid solution. However, the additional plumbing adds restriction to overall system, which adds in-efficiencies.
Quote:
Originally Posted by Boss330
You are just kidding, right  ?
Renault's F1 engine department clearly doesn't agree with your statement anyway...
Lag and "torque wave" are two very different things. A NA engine will, for all intents and purposes, have instant torque delivery. When opening the throttle there will be, within milliseconds, a delivery of 100% of the available torque at that rpm and throttle opening.
A turbo engine will have the same, millisecond, delay before it delivers the unboosted maximum torque. When the first combustions at that level have taken place, the extra amount of exhaust will come into the exhaust manifold and start the process of spooling up the turbo. Depending on the operating stage of the turbo, this spooling up process might have to include first stopping the process of spooling down (turbo is loosing rpm's), then starting to accelerate the turbo up to it's maximum rpm and boost for that engine rapm and throttle opening. The turbo's compressor side also has to start the process of compressing the air, routing it through the intercooler and the necessarily longer intake tubing before the compressed air reaches the intake manifold and then enters the engine. Just the extra volume and length of the intake side of a turbo engine MUST produce a delay between throttle input and power delivery. Hence why even on a F1 engine with a motor to spool up and down the turbo, the manufacturer states that they cannot get as rapid response as a NA engine. By design, a turbo'd engine can't have as fast response as a NA engine.
A turbo engine will, a few milliseconds after throttle opening, only deliver a certain percentage of it's maximum power delivery for that rpm and throttle opening. That percentage depends on how much of it's power is made under boost. The higher the boost, the higher the percentage of power is made under boost. A engine that has a boost pressure of 0,5BAR (low boost) will typically make around say 40-50% more power under boost than it does without boost.
So, a turbo engine that boosts at 0,5BAR will only deliver 2/3rds (66%) of it's maximum power (at that rpm) a few milliseconds after the throttle has been opened fully. It then takes the added time of completing the combustions which creates the exhaust gases needed for spooling up the turbo(s). Then the turbo starts generating boost, delivering the air through the intercooler and intake plumbing before entering the intake manifold and finally the engine. That is turbo-lag my friend and is not remotely comparable to a NA engine.
Yes, a turbo engine will deliver more torque than a similarly sized NA engine. But where the NA engine delivers 100% of it's available power just a few milliseconds after throttle input, a turbo engine will only deliver maybe 50-60% of the available power in that same time frame, and then you have a delay before the turbo has reached max boost and you have 100%. It's kind of like a two stage power delivery. As explained above, it takes added time to reach 100%. There is currently no way around this, not even the clever F1 engineers have been able to eradicate it completely...
A NA engine cannot have lag from idle to redline, it constantly delivers 100% of available power at any given rpm and throttle opening. A turbo engine cannot, and does not, replicate this quality. Not even next years F1 engines!
And, just to reiterate, I love the torque wave a turbo engine delivers. It's addictive and much more useable in daily driving than a high revving NA engine that needs high rpms to deliver the goods.
And BTW, the beauty of the MGU-H unit is that it can start spooling up the turbo at the instant the throttle is opened. The turbo doesn't have to wait for the exhaust gases to start the spooling up process. This saves some time and boost production starts allmost immediately. Obviously not instantly as the compressor side has to compress the air and then flow it through the intercooler etc. But at least the turbo can respond immediately to throttle input  The delay (lag) on the exhaust side of the turbo has been eliminated/minimized, it's now the compressor side of the turbo that is the cause of delay (as the turbo's compressor side cannot go from 0BAR boost pressure to 3,5BAR immediately. Air is compressible (unlike most fluids) and it therefor takes time to build pressure as more volume of air is needed to fill up all tubing/intercoolers etc before the desired pressure can be reached. The turbo has to pump air to build pressure, which takes time). |
Regarding the NA being smitten with lag all the way to redline is not difficult to understand. I'll explain....
Lets take an S65 type engine, and add a turbo charger system to it. If we do it right, even if we disconnect the wastegates so no boost is ever achieved, it still runs like an NA s65, all the way through the redline. Anyone driving this car not knowing there are turbo's attached to the engine, would comment on it feeling exactly like an NA s65.
Now we connect up the waste gates allowing the engine to build boost. All of a sudden there is 'lag' right? Well not comparing it the the NA s65, the turbo doesn't make things any worse than before, the torque curve compared to an NA version is identical.....but yes there is a delay before an even greater hit of torque takes place.
Comparing a small displacement turbo engine with a similar power but larger NA engine is when you notice the lower off boost torque.
You do correctly paint the picture of lag, but lets just visit some factors....
A turbo is a small device capable of accelerating very quickly in the right conditions. It is an air pump. When boost pressure in the inlet tract is low, you are powering a device with no load on it, this allows it to spool ridiculously fast.
Secondly, common myth is that a turbo has to be at 140,000+ rpm before it makes any boost. Totally incorrect, it depends on the Turbo airflow charts, compressor sizes, etc, however, more commonly the truth is they can start producing positive boost at half those revs. Now you might think that 70-80k revs is still high enough to take time to reach? Not really, the turbo is never stationary, even at IC engine idle it rotates at a few thousand rpm (no load on the pump remember). When WOT is reached, the energy going through the turbo increases very rapidly. Think 335i producing full torque from 1200rpm. This indicates that full controlled boost is reached by 1,200 engine revs.
that doesn't leave much delay for lag, and in fact I would be surprised if the turbo was even below boost threshold at idle. The higher the IC engine is above boost threshold, the lower the lag due to the high energy in the exhaust. A 335i will require waste gate opening from as little as 1200rpm to vent excess exhaust energy.
In summary, yes lag exists, but is grossly over exaggerated in modern systems. You might get a 0.25 second delay before full torque is achieved for the revs, compensated for by increased torque shortly after.
Coming back to F1, in the 80's turbo lag was massive, lasting seconds. But remember this, they realised that pumping enough air volume at the mandatory blow off valve, overloaded it in such a way that you could boost the engine to higher pressures than IIRC the 2 bar blow off valve allowed. This means the turbo was grossly oversized for the application. They used to run at something like 5 bar. That will not be the case for modern F1.
Last little point on competition, there isn't a single race class where (when allowed) a turbo charged engine is not the fastest type. i.e. there are no race series where NA rules over Turbo.