This simple animation is a bit confusing, but in practice, exhaust gases are only used to spin the air pump connected by a shaft to pressurize the in-coming
air/fuel. Exhaust gasses are not sent back in by the turbo to be burnt again.

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Some exhaust gases are sent back, but it has nothing to do with whether the engine is turbo-charged... it's called an EGR (exhaust gas recirculation) valve, and it's nothing new (it's an emissions control device).

There are many types of 'efficiency'. The problem with turbo motors is they really aren't that efficient in MPG terms when they are making boost... they are really only higher in 'volumetric' efficiency (higher power/displacement and/or power/weight ratio), though they do make power more consistent between low and high altitudes (which can mean you get by with a smaller engine rather than having one that's big enough for high altitude but bigger than needed at low altitude). Power being similar between a turbo motor and a naturally aspirated motor of larger displacement, they aren't usually very different in MPG (and sometimes the turbo motor is worse). One thing that hurts many gasoline turbo motors is that to avoid detonation with boost, they usually lower the compression ratio of the motor, which hurts overall MPG, though with diesel turbos it's not usually an issue (because they detonate by nature, though you still don't want them to ignite too early in the compression stroke).

Stay out of boost, and you can get better MPG, but you really have to drive like grandma. Many modern turbo motors utilize small turbos that are in boost a lot more often (gives better throttle response), so it's almost impossible to avoid boost.

So overall the green effect of turbo motors is subtle.

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Depends,

If the beefier more efficient motor is merely being used to justify a beefier and heavier car.

But yes, in general more efficiency is a good thing.

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"Power being similar between a turbo motor and a naturally aspirated motor of larger displacement, they aren't usually very different in MPG (and sometimes the turbo motor is worse)."

Exactly, since the turbo motor is smaller, it'll consume less gas than a larger gas engine. But, both will still have the same amount of power too. Right?

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@ Lascelles -

conventional turbo systems compress only fresh air. The traditional objectives are increased specific power and reduced power loss at altitude. Core efficiency is improved because engine torque will go up rather than down during the intake stroke. However, the higher absolute pressure levels during combustion also increase friction between piston rings and liner, so internal losses are higher. Moreover, the geometric compression ratio of a turbocharged gasoline engine is lower than that of a naturally aspirated design to avoid engine knock.

You can also use turbocharging to improve fuel economy rather than absolute power. Reducing displacement per cylinder - or preferably, cylinder count - (a.k.a. downsizing) means you can reduce throttling losses in part load. Add cam phasers and gasoline direct injection and, you can sharply improve scavenging at low RPM: by injecting the fuel only after the valves close, engine-out HC emissions are unaffected if some fresh charge is blown through.

If you apply a small turbo, a variable geometry turbo (VGT) or a two-stage system, boost levels can be high even at low RPM. If the shift points in the transmission are tuned to take advantage of this, the engine can deliver low-to-midrange power at reduced speeds and higher loads, i.e. reduced coolant and friction losses.

As a rule of thumb, downsizing by 25% using a single turbo stage can preserve rated power and improve fuel economy by 10%.

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In a diesel engine that utilizes high-pressure externally cooled EGR to reduce engine-out NOx emissions, hot exhaust gas is siphoned off the exhaust manifold prior to reaching the turbine. After cooling, this portion of the exhaust gas is fed into the intake manifold between the turbo's compressor and the engine.

One upside of this setup is that the high temperature differential between exhaust gas and coolant (typically air, sometimes liquid) is high, so the EGR cooler device can be small. Another is that EGR rate control is not subject to significant delays as the length of the associated ductwork is short.

The primary downside is that the EGR valve - a highly precise device - is exposed to high temperatures. Also, mass flow to the turbo is reduced. Note that the pressure gradient between the manifolds is high enough only when the engine is at high load, so high-pressure externally cooled EGR is not possible across the whole engine map.

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Ricardo and others are now developing low-pressure EGR in which exhaust gas is siphoned off downstream of the turbo, oxidation catalyst and DPF. The cooled recirculate is mixed with fresh air upstream of the compressor. The EGR cooler is larger and EGR controller dynamics are worse.

The upside is that this system can be applied to gasoline engines and enables the use of variable geometry turbine (VGT) turbos made from affordable materials. As explained above, this is important for fuel-saving downsizing concepts.

The pressure gradient is sufficiently high to support 20-30% EGR at virtually all engine speeds. That means throttling losses in part load can be further reduced. Turbo lag is reduced and turbo efficiency improved by the greater available mass flow. Tailpipe emissions standards still met using a regular inexpensive three-way catalytic converter.

Moreover, because high EGR rates lead to cooler engine-out exhaust temperatures, there is no need to enrich the mixture to protect the turbo and catalytic converter at high load. This improves emissions and fuel economy in real-world driving beyond the narrow load profiles of the official drive cycles.

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Conclusion: gasoline engines can now be made more fuel-efficient by turbocharging only because of advances made in turbodiesel technology over the past 15-20 years.

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@ Lascelles -

Toyota's Hybrid Synergy Drive is a compound hybrid, i.e. it splits engine power into a mechanical and an electrical transmission path and recombines them using a single planetary gear set.

A true series hybrid (cp. GM E-flex, Volvo Recharge, Citroen Cactus PHEVs) has no mechanical transmission path at all. Since the electrical path is quite inefficient by comparison, a net gain in fuel economy is possible only thanks to the large buffer batteries that shield the engine from load peaks. It can therefore be much smaller and always operate efficiently at moderate RPM and high load - or not at all.

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Typically, turbochargers have not done gas engines a whole world of good in fuel economy. The problem is you have to reduce the compression ratio to reduce knocking and that reduces efficiency.

It's better on Diesels, because they don't have to reduce compression ratios. New HCCI engines may be able to bring some of these benefits to gas engines.

Perhaps the biggest problem with turbos as mpg devices is that turbos put more air into the engine. Putting more air into the engine requires you put more fuel in to keep a stoichiometric ratio. More fuel burned hurts mpg.

If a turbo lets you put a lot smaller engine in a car, it can help with efficiency, due to reduced total friction and reduced mass (which hurts in stop and start driving). But honestly, taking the same engine and just taking the turbo off will produce even better economy. In that way, perhaps the gas turbo's best attribute is it helps make the car accelerate better so people don't turn their noses up at it. An efficient car doesn't save any gas if no one buys it.

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