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Author: Subject: Engine downsizing and turbos
Nitrogeno25

posted on 29/7/14 at 04:32 PM Reply With Quote
Engine downsizing and turbos

I was thinking about the fuel maps on turbo cars and they always use lower afr (more fuel) when on boost.

I understand that better torque allows less revs with better gearing, but then at low revs and low load the turbo is not doing much.

Is the turbo a real advantage (talking about fuel economy)?

What do you think!

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me!

posted on 29/7/14 at 04:57 PM Reply With Quote
at low revs and low load the turbo is not doing much

This is the point- modern 'downsized' turbo engines don't boost over the majority of the operating range, so you've basically got a smaller NA most of the time. If you put your toe down of course they will boost, and make the power and torque of a much larger engine.

I'd guess this means that it runs with a wider open throttle most of the time, but boost very limited using wastegate control. This would be more efficient than a larger engine running heavily throttled due to pumping losses. And the fuelling won't be that far from stoichiometric in a modern turbo engine- catalysts lose efficiency quickly either side of 14.7:1, and emissions is king at the moment.

Of course an aftermarket turbo setup is very different. Lots of boost, lots of fuel, and lots of choo choo noises :-)

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Nitrogeno25

posted on 29/7/14 at 05:06 PM Reply With Quote
quote:
Originally posted by me!
at low revs and low load the turbo is not doing much

This is the point- modern 'downsized' turbo engines don't boost over the majority of the operating range, so you've basically got a smaller NA most of the time. If you put your toe down of course they will boost, and make the power and torque of a much larger engine.

I'd guess this means that it runs with a wider open throttle most of the time, but boost very limited using wastegate control. This would be more efficient than a larger engine running heavily throttled due to pumping losses. And the fuelling won't be that far from stoichiometric in a modern turbo engine- catalysts lose efficiency quickly either side of 14.7:1, and emissions is king at the moment.

Of course an aftermarket turbo setup is very different. Lots of boost, lots of fuel, and lots of choo choo noises :-)


I undestand that, a small engine with low load is more efficient, but on the other side, they have to reduce compression ratio for on-boost situations so the low load is compromised, Am I right?

Cheer!

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me!

posted on 29/7/14 at 05:18 PM Reply With Quote
I think they run quite high compression ratios (10+) and lower boost. High compression ratios are helped by direct injection as well- the cooling effect of the fuel being vaporised in-cylinder helps reduce knock.

This could all be b*llocks of course- I work in the motor industry but I'm a long way from being an engine designer or calibrator!

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baz-R

posted on 29/7/14 at 05:26 PM Reply With Quote
a turbo is effectivly recycling lost energy so yes its more econmical but sadly the effect only realy works well on a diesel as there is no throttle butterfly and diesel will run realy lean without any issues.

when you apply a turbo to a petrol engine you get a few problems as you cannot run lean and the gas flow(throttle) is restricted to control the engine power.

the newest cars are now using econ based turbo designs purly to save fuel like the ecoboost engine but thease run small turbos that run harder at a much lower revs and are coverd with valves and sensors and could well be very tricky to convert

i have been looking into what would be needed to run a ecoboost 1l unit as a convertion and the suff needed would mean a bespoke aftermarket ecu as there is lots and lots of bits and bobs to control.

i can build the ecu with the extra circuits needed but i need to get hold of an engine to go any ferther.


anyone got a free 1l ecoboost thay would like to donate to me?

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coyoteboy

posted on 29/7/14 at 11:26 PM Reply With Quote
Turbochargers increase efficacy, not efficiency.

The gains come from having a 1.4 that behaves like a 2 litre with only a hint of boost. As mentioned, low boost can be achieved with sod all penalty in off-boost performance.

Regards AFR - it's only under high-to-WOT conditions that you see significantly different AFRs. For example, my tin top uses closed loop feedback to stay at stoic until 6psi of boost, when it drops to 13:1. That's not far off a N/A of the same era.

[Edited on 29/7/14 by coyoteboy]





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MikeRJ

posted on 30/7/14 at 08:55 AM Reply With Quote
quote:
Originally posted by coyoteboy
Turbochargers increase efficacy, not efficiency.



Turbochargers can actually increase thermal efficiency. Because you have a physically small combustion chamber densely packed with air and fuel, the flame speeds are higher, which means more of the heat energy goes into pressure increase rather than being absorbed into the cylinder walls and combustion chamber. Obviously this is at least partly offset by increased pumping losses, but there can be a useful overall improvement.

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coyoteboy

posted on 30/7/14 at 09:52 AM Reply With Quote
BSFC is, without exception as far as I'm aware, higher in turbocharged/supercharged cars (I'm only counting petrol, I've not worked on numbers for diesels) than N/A. They're more effective, but less thermodynamically efficient.

[Edited on 30/7/14 by coyoteboy]





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DW100

posted on 30/7/14 at 10:57 AM Reply With Quote
Part of the change is to do with the increased level of technology and the use of direct gasoline injection.

Under some circumstances they work like a conventional petrol, with the throttle controlling airflow into the engine and what they call a homogeneous fuel mixture (like standard petrol where fuel and air are mixed throughout the cylinder)

Under other circumstances they work more like a diesel, with the throttle wide open and what they call a stratified charge. Fuel is injected as a small ball around the spark plug and surrounded by plain air. The spark ignites the mixture around the plug but the air around it insulates it. In this way overall fuel ratios of up to 50:1 can be used along with higher compression ratios leading to better fuel economy.

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coyoteboy

posted on 30/7/14 at 12:58 PM Reply With Quote
Charge stratification does indeed help a lot, but it will help both N/A and turbocharged similarly. It may disproportionately help forced induction by keeping fuel away from hot spots allowing more knock control but the last time I spoke to a chap in the field they said they were a long way from perfecting it and current implementations are crude.





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morcus

posted on 30/7/14 at 04:45 PM Reply With Quote
Call me cynical but I was under the impression the biggest gains were in official Fuel Consumption and Tax?





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coyoteboy

posted on 31/7/14 at 12:01 AM Reply With Quote
You can't argue the vw twincharged powertrain (1.4) doesn't produce pretty much better results across the board compared to its 2 litre counterpart though. And isglacially more appealing.





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