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!
