Dull technical-ish 'how's a turbo work?' question

I know roughly how a turbo works (exhaust gases spin a turbine, which forces more air(?) into the chamber, making a more efficient burn). Or something like that.

My specific question is; do they spin when an engine is on ‘over-run’, i.e. the engine is revving but no throttle applied, for instance when engine braking down a hill?

You’ve answered your own question. The engine only puts out turbo spinning levels of exhaust when you’re giving it the juice, when you’re rolling down the hill it’s essentially on tickover.

That’s what I thought 🙂

Some high boost petrol cars have a pressure release valve that goes ‘tsss’ when you lift off to release the pressure and stop the turbo spinning and putting more air in, which then gets heated and comes out of the exhaust, keeping the turbo spinning even more and so on.

Other cars have a little speaker that goes ‘tsss’ to make people think it’s got a big turbo.

Yup, as above, it only does anything when the engine is working a bit, on very light throttle, low revs or over-run when there isn’t much air passing through the engine, the turbo will coast down a bit. It’ll still be spinning but it won’t be compressing the inlet air, just “free-wheeling”.
When it’s working it doesn’t increase efficiency so much as cram more air into the cylinders, which allows you to burn more fuel, which makes more power.

Some high boost petrol cars have a pressure release valve that goes ‘tsss’ when you lift off

The infamous Scooby dump valve?

No fear of that with mine, it’s a diesel for a start. Although, as we discovered, it does seem to run quite happily on petrol 🙂

When it’s working it doesn’t increase efficiency

Yeah it actually slightly reduces efficiency by obstructing the exhaust a little bit. However, in diesels it allows decent performance from a relatively smaller engine which is then correspondingly more efficient when not on boost than a larger one would be. I think, anywyay.

Yeah it actually slightly reduces efficiency by obstructing the exhaust a little bit. However, in diesels it allows decent performance from a relatively smaller engine which is then correspondingly more efficient when not on boost than a larger one would be. I think, anywyay.

Pretty much. On Petrol engines with a significant sized turbo (scoobies, mitsubishis,RS fords etc) it reduces efficiency because the staic compression ratio of the engine is lowered to allow more boost to be run, which gives lower efficiency and less power off boost but allows the turbo to be run harder and produces more power. This is a big part of the reason for the traditional nothing-then-lots turbo delivery as these engines are only about 1:8 compression so they are utterly terrible off boost, then suddenly the output more than doubles as the turbo comes in.
In more mass market cars where they are using the turbo to allow a 1.4 to make modern golf move properly the efficiency is far better as the amount of boost used is far less so the basic engine doesn’t have to be de-tuned to prevent it blowing up, and you actually do get the benefit of lower weight and smaller internal losses with the smaller engine.
The actual reason for dump valves on road cars is not quite what it’s popularly believed to be. When the throttle is fully closed on a pertol engine the turbo can’t keep

putting more air in, which then gets heated and comes out of the exhaust, keeping the turbo spinning even more and so on

as it’s trying to push air into a dead end, there’s no path for air to flow through the engine with the throttle shut. What happens without a dump valve is that the throttle closes, the momentum of the turbo pressurises the inlet manifold behind the throttle butterfly and then because the exhaust gasses have stopped flowing too (no air in = no air out) the turbo stalls as all that compressed air in the inlet manifold tries to come back out. That’s pretty hard on the turbo as it goes from spinning very fast one way to stopping and probably spinning slightly the other way in a fraction of a second, it also pushes the spindle of the turbo in the other direction against the bearings, which most turbos aren’t designed for. Rally cars don’t have dump valves and just let this happen as the turbos they use are designed for it, that’s what the “chiirr” noise is, not the dump valve, not a waste gate, it’s the turbo stalling. So dump valves are simply to protect the turbo and if anything reduce performance. Almost all turbo charged petrol road cars have one, the reason you don’t hear them is that they dump back into the inlet manifold not just out under the bonnet.

Almost all turbo charged petrol road cars have one, the reason you don’t hear them is that they dump back into the inlet manifold not just out under the bonnet.

Well you learn something new everyday…

robinlaidlaw – Member

^^^ This man speaks the truth!

Almost all turbo charged petrol road cars have one, the reason you don’t hear them is that they dump back into the inlet manifold not just out under the bonnet.

They’re normally given a different name, on my car it’s called a ‘recirculation valve’.

it’s trying to push air into a dead end, there’s no path for air to flow through the engine with the throttle shut.

Ah yeah now you say that, that is what I’d originally read and forgotten.

However surely there’s also an advantage in terms of throttle reponse in not stalling the turbo…?

They’re normally given a different name, on my car it’s called a ‘recirculation valve’.

Yup, or as fitted to un-modified Imprezas, a recirculating dump valve

O.K, I need to get off this thread before I brain-dump everything I learned about turbo engines playing with Scoobies and hanging out on Scoobynet and bore everyone to death. 😀

I love talking about engines and stuff 🙂

I once had to replace the turbo on my old Passat. It wasn’t that complicated though 🙂

They’re normally given a different name, on my car it’s called a ‘recirculation valve’.

And come in atmospheric (whoosh tissssh!) and non-atmospheric varieties.

However surely there’s also an advantage in terms of throttle reponse in not stalling the turbo…?

Yes and no, it probably depends on the size of the turbo. With a big turbo, the delay for the turbo to spin back up is more than compensated for by the fact that if you don’t dump the pressurised charge, it’s still there waiting to rush into the engine when you open the throttle again, which in turn helps to spin the turbo back up.

The developent of hybrid cars presents all sorts of opportunities for energy recovery via electricity generation. I thought it’d be a good idea to have a generator powered by a turbo to replenish batteries.

molgrips – Member

The developent of hybrid cars presents all sorts of opportunities for energy recovery via electricity generation. I thought it’d be a good idea to have a generator powered by a turbo to replenish batteries.

This is in the specs for the 2014 F1 engines, in addition I think they recover heat from the turbo as well.

The recovered energy can be used to re-spin the compressor to reduce lag, or pumped back into the motor which is attached directly to the crankshaft.

The most interesting engine improvements I’ve seen recently have been the 6-stroke petrol/steam engine concepts and the keoeonenosigsesgs electric valve train which means no camshaft, belt/chain and completely variable valve timing.

Yeah, there are several demonstrators out there at the moment and they seem to do a very good job – definitely something that will become more widespread in the coming years.

I’d have bodged one myself from an old car but I think a normal turbo would spin too fast for me to be able to engineer a generator onto it 🙂

Also electrical inductive dampers feeding energy back to the battery.. and a steam turbine driven from the coolant, instead of a radiator.

The most interesting engine improvements I’ve seen recently have been the 6-stroke petrol/steam engine concepts

I’ve seen this, it’s fantastic. Is it being developed? I hope so.. maybe that’s why it’s all gone quiet in public?

I’ve also heard about electric valves. Presumably there’s a lot less friction too? Fiat’s twin-air is a small step in that direction, isn’t it?

In more mass market cars where they are using the turbo to allow a 1.4 to make modern golf move properly the efficiency is far better as the amount of boost used is far less so the basic engine doesn’t have to be de-tuned to prevent it blowing up, and you actually do get the benefit of lower weight and smaller internal losses with the smaller engine.

Are you not going to be running a lot of boost as it will be a small turbo on an equally small engine.

The advances in engine and turbo technology account for there being little or no boost threshold compared to your Subaru of old, which may also have had a larger turbo shoved onto it than standard 😉

I remember driving a standard Rover Coupe Turbo with the boost modulation valve, which slowly increased the boost as the revs increased on a T25 turbo, fairly smooth power delivery. Replace this with a T28 and a ball and spring boost controller which would give 12psi straight away and it was arm wrenching stuff.

molgrips – Member
I’ve seen this, it’s fantastic. Is it being developed? I hope so.. maybe that’s why it’s all gone quiet in public?

I’ve also heard about electric valves. Presumably there’s a lot less friction too? Fiat’s twin-air is a small step in that direction, isn’t it?

I can’t find the page I was reading about the 6 stroke engine, but I *think* there was a working concept which had no supplementary cooling system.

Here’s a link to the F1 spec, turbo and ers is about half way down:
http://somersf1.blogspot.co.uk/2013/01/2014s-power-units-16-v6-turbos-with-ers.html

And here’s a video about the keoeoneenegsisg valve train:

edit: here’s a bit about the six stroke, from 2006
http://www.autoweek.com/article/20060227/free/302270007

Thanks robinlaidlaw for the informative post 🙂

I quite like the subtle sound of the dump valve under the bonnet of our car. It’s a Forge 007p, upgraded from the stock one.

but I *think* there was a working concept which had no supplementary cooling system.

The programme I saw he had a working single cylinder stationary engine on a bench, looked about the size of a 50cc moped engine. It had been an air cooled one I think and you could hold the sides when it was going (and doing work) and it was just warm to the touch.

Brilliant video! I knew that the servo valve idea had been around for a while, so I saw the bench test and was about to click away – but then they showed the actual car they had working on it – fantastic!

I’ve already had most of those ideas myself, I just don’t have the engineering capacity to make them so I’m glad someone is!

Also very good to see someone putting the profits from selling stupid supercars into something really useful for everyone.

Are you not going to be running a lot of boost as it will be a small turbo on an equally small engine.

Well, you can get quite a decent amount of power out a decent N/A 1.4 these days, so comparatively, no you don’t need all that much boost, particularly when you aren’t expecting anyone to try to get more, for rallying / track use etc so you can run the stock set-up much closer to the actual limits. It’s getting lots of mid-range torque that makes these fairly small engines work in fairly heavy cars and you don’t need to push the turbos hard to do that. Plus you are looking for efficiency, not outright power, so you run higher compression, more advance and less boost, whereas for highest outright power you tend to go with lower compression, less advance (too much tends to bring on pre-ignition and detonation) and run lots of boost to just cram a whole lot of fuel and air in there. It’s not efficient but it produces the most power when that’s all that matters.

The advances in engine and turbo technology account for there being little or no boost threshold compared to your Subaru of old, which may also have had a larger turbo shoved onto it than standard

Very true, but it’s rather fun to have the massive sudden kick in a 4wd car tht easily puts the power down, so often they are deliberately tuned to do that as the huge surge on mid throttle / mid revs makes them feel lovely for point and squirt roads. It’s entirely possible to achieve much more linear response with variable vane turbos, twin turbo set ups and so on but there’s a lot more to go wrong and it costs a lot more to do.