Thursday, July 18, 2019

Salt Slush Summer Reading. Chapter one - Matching part 2 and The Turbo Loop

It’s time for our next blog post about engine boosting.
Pour yourself a cup of coffee and stay focused, because we’re going to continue our tour in the high density landscape, where there's good and bad, heaven and hell, eternal fire and cold fresh air. All at the same time. You got it, this time it's all about turbos…
But before we move in to details let’s make one thing clear; Turbo matching is always a compromise. Therefore, it’s a very good idea to first of all determine what kind of engine you want and how the engine & vehicle are supposed to be used.

If you’re looking for a fast street machine with no (or minor) turbo lag you need one type of turbo matching strategy, if you’re into drag racing you’ll need another matching strategy and if you’re Drifting, Time Attack you need something else.

Nevertheless, the physics are all the same. Let me try to explain.

It may be obvious to you all, but it is important to keep in mind that the turbine and the compressor side is connected and in between there’s a gas exchange system. Which means that what happens on the cold side (compressor) effects the hot side (turbine) and vice versa.
With the two pictures below I have tried to illustrate the compressor, combustion system and turbine interaction. (click to make them bigger).

Let’s start with the good and desired turbo loop in which the back pressure before (and after) turbine is low, the charge cooler is efficient and the compressor efficiency on top level. In this preferred case the pumping losses will be low (due to the low backpressure before and after turbine) and charge air temperatures too, since the compressor efficiency is on a high level. When we are in this Thermodynamic Happyland the risk for knock and misfire will be limited too and spark advance will come close to MBT (Maximum Brake Torque) at peak power.

The only problem with this gas exchange Nirvana is that it requires a too big turbine (flow capacity) and too big compressor and consequently a substantial turbo lag. A too big turbo with too much flow capacity (i.e. power) normally brings low torque at low and mid engine speeds and long time to spool up. It helps a lot with ceramic ball bearing and titanium-aluminum turbine wheel (ref. Borg Warner EFR), but this way is still not a good way to go.

Before we jump to conclusions, we need to look at the dark side too. I’m talking about the turbo hell, in which pressures are high and temperatures rising without power and torque gain.

How do we avoid ending up on the dark side of boosting with knock, misfire and engine failure? It’s actually easier than you think to end up on this side when working with high boost turbo engines. A too small turbine house, an inefficient or wrongly matched compressor or a too high compression ratio and you're in the negative loop.

Example: At the end of any compressor map the efficiency drops significantly and the charge air temperature is climbing as fast as the efficiency drops. At this point, it is hard, even for a highly efficient intercooler, to handle the high temperatures from the compressor. In addition, the low compressor efficiency steals power from the turbine and increases the backpressure before turbine. High air inlet temperatures, hot exhaust gases that is pressed back in to the combustion chamber due to the high back pressure, makes life hard for the combustion system. 
A well designed fast burning DI engine have a chance to cope with this situation reasonably well, but a PFI (Port Fuel Injection) engine will start to knock or misfire. The way to handle this problem is clear; retard the ignition. However, retarded (late) ignition makes the combustion less effective and more boost pressure and flow to keep the same power output is needed. Which makes thing even worse…

So how to match a turbo and avoid the worst mistakes? Here's our check list...
1. Deiced what kind of engine you want. [Peak Power, Purpose, Characteristic]

2.  Calculate / Estimate air mass flow for the desired power of the specific engine.

3. Estimate boost pressure for the specific power.
[rule of thumb: 110-120hp/Litre (61 in^3) generates approximately 1-1,1 bar (14,5psi) boost pressure (relative) in your PFI engine.
4. With estimated max flow and boost pressure at hand, choose compressor map.
Advice: Always leave some flow margin. 10% if it’s a traditional turbo and more margin if it’s a ceramic ball bearing Borg Warner EFR with TiAL turbin wheel. Reason: Margin lower the stress levels (temperature and knock)

5. Keep pressure losses in inlet and exhaust system low. It helps a lot.

Keep on Boosting!


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