Turbo Stuff

TL;DR Turbo-curious

The previous 150 or so posts probably left little doubt about the fact that I'm no mechanic. With that came basically no prior knowledge about the ins and outs of a turbocharged engine.

In a nutshell, both turbochargers and superchargers push more air in the combustion chamber than atmospheric pressure alone would allow. A turbo is driven by the exhaust gases, while a supercharger is powered by an accessory belt, stealing engine power from crankshaft (in basic terms : just like the alternator, or better yet, like an AC compressor).

With a boost pressure of 1.3 bar (measured through the OBD port), the Caterham 170 sees an absolute pressure of 2.3 bars in the intake manifold when full boost is achieved.

That forced induction basically turns the puny 658cc block into a 1513cc one. Who's laughing now? Compared to the 0.3  bar boost pressure on supercharged Lotuses, that's quite something.

Pushing air then... yes.  Like a Nitrous oxide (NOS) kit, it's all about providing more oxygen to the combustion chamber.

In both instances (turbo/super-charging or NOS), additional fuel has to be added too. Failure to provide enough fuel leads to a lean mixture that will burn the engine, literally. A good example is SuperfastMatt who raised his NOS pressure and forgot about changing the fuel map in the ECU of his land speed car.
Note that compression ratio is kept lower too (like 9.5 instead of 11 or so).

With fully digital machines, it's easy enough to have an adequate fuel map, one that is a bit on the "rich" (safe) side when running at full boost/power. With carburetors it was probably trickier, with additional fuel injection and all.

Anyway, on the Caterham, all is managed by the ECU : solenoid valves are the interface to vacuum actuators. Direct injection, ignition timing and inlet valve timing are fully digital too.

Ok, but what are we really controlling with that ? Put simply, there is the "waste gate" on the hot side (turbine in the exhaust), and the "dump valve" on the cold side (compressor wheel that feeds the air intake manifold).

Those are responsible for setting the air/gas circuits in the proper configuration to reach and maintain the expected intake manifold pressure at any moment.

They are also responsible for the typical whooshing noises of the turbo.

In "boost" configuration, the waste gate remains closed, directing all the exhaust gases through the turbine, exiting to the catalytic converter, meeting the oxygen sensor along the way, and continuing to the silencer. On the cold side, fresh air enters the air filter, goes through the mass-air-flow sensor and enters the compressor wheel. That compressed air gets hot due to the compression, then flows through the (rather large) intercooler before reaching the intake manifold, throttle valve, and finally the engine head.

As soon as you lift the gas pedal, the throttle valve closes. At that moment the waste gate opens to let the exhaust gases bypass the turbine, while the dump valve opens to shunt to exit of the compressor to its inlet. That's when you get all the lively noises from the turbo. Mine sounds like a bus releasing pressure from its pneumatic brakes.

So, yes, the Caterham has a blow-off valve (dump valve). It's of the "recirculating" kind, meaning no air is released to the atmosphere, all remains contained in the intake circuit. That's the only way to manage the fuel ratio with an ECU that reads the MAF (mass air flow) sensor : if you let excess pressure out by releasing air, you would foul the engine with excess fuel as soon as you give some gas back.

On mine, the SCS-Delta ECU works with the intake manifold air pressure and temperature, the MAF is not used at all. I could have a valve that releases everything to the atmosphere or an hybrid one with a bit of both, and the ECU wouldn't care. There are pros and cons to blowing air off, and they are mostly negative. The only "plus" is more noise. Mine is gnarly enough as it is with its OEM recirculating valve.

If you don't release the pressure from the intake, you would stall the compressor. That gives a nice "stututututu" noise, known as turbo-flutter. It's not the best for reliability. Stock Caterham 170 have a bit of gentle flutter, I assume they open the dump valve a bit later than my ECU does with the current configuration.

Not opening the waste gate on the hot side is probably even worse, as we are entering anti-lag territory (basically injecting fuel to be burned outside of the combustion chamber - that's super hard on the turbo).

In the first year, I was surprised not to hear whooshing while I could hear it in some Youtube videos. I had the gentle flutter (think of a sleeping owl), but no whoosh. It's only very recently that I became the second passenger in Christophe's 170S and could hear that his car indeed produces gentle whooshing noises as well. Mine is now very different as I have no flutter at all, but the crazy whoosing described above.