MIT Researcher Explains Why Fuel Economy is Still Low Despite Advances in Fuel Efficiency

Absent an engine compression mode "switch", Turbocharging a passenger car engine ALWAYS means sacrificing FE 99.98% of the time in return for POWER during that remaining 0.02% of the time. You cannot(***1) have a respectable level of turbo boost with an engine that is already at the high compression limit, BEST FE, for off-boost operation.

Take one of Ford's EcoBoost gas-guzzling engines and mill the head to get the standard DFI compression ratio of 12:1 and watch the cruise FE improve by at least 10-20%

That level of FE improvement for 99.98% of passenger car driving time becomes a VERY serious number.

***1. Except:

If you're still serious about more EcoBoost power then add a "clutched" SC and a refrigerant-to-air(***) intercooler. Full boost pressure but at 50F and what might the effective CR of the engine become..?

*** more likely the actual intercooler would be a "standard" liquid-to-air design. 50F liquid (anti-freeze) previously/continuously chilled via A/C refrigerant and stored for (later) use.

 

But you can have a respectable level of boost for most driving conditions. It all comes down to how you want to use turbocharging: Do you use it for high performance, or do you want to get more power from a smaller engine. Traditionally people think of "turbo" and they immediately think high-performance. Turbocharging can also be used to get acceptable performance from an otherwise too-small engine.

Tom

 

The poor quotes are generating confusion to what the research is about.

http://web.mit.edu/newsoffice/2011/cars-on-steroids-0104.html

Its about environmental economics - public policy.

Many have said this, cafe standards by themselves will not reduce oil consumtion as much as economic incentives of raising gas taxes.

He did not examine the mileage cost of safety and environmental regulations. An overwhelming majority of cars on the road were purchased when inflation adjusted gas prices were lower than 1980.

 

If you wish to make use of a turbocharger you MUST provide a means for driving its turbine. If you can do that without "wasting" that same level of energy off-boost then you are my hero.

 

Certaintly interesting, but not shocking. Most people do not care about FE, and the moment gas dips below $3 they are all out buying new trucks. Then they are amazed, and angry when gas goes up again. To me it is is now shock. However I am not to concerned with it myself anymore though. I say just make it $10 tomorrow. Eventually they will get the point. I think in November when I fill the Volt up again it is going to be a mighty expensive $30 to refill if premium stays at $3.70. Otherwise on track to keep paying $0.03 a mile for electric. Can't wait for you guys to join me with the PiP. Way I see it let others drive what they want. Eventually it will catch up with them.

 

Turbos are using the exhaust to drive them. Which, by definition, a waste product. Unless the vehicle has an exhaust driven generator for electricity(not available on consumer vehicles) or an exhaust heat recovery system(only on an Atkinson engine which is taken as incompatiable with a turbo), the exhaust is just being thrown away anyway.

Turbos do not have to run at a boost that requires detuning out of boost or DI.

Engine efficiency, while important, is just one facet of vehicle efficiency.

 

...

 

So what you really want is a Carnot cycle engine. That way you won't have any power left for the turbine.



I don't think anyone here said or implied that turbocharging was the best and most efficient possible internal combustion engine. I know for sure that I didn't. All I said is that turbocharging can be used in a more or less steady state to allow a smaller engine to produce more power, as opposed to occasional high end boost for performance. That comment remains correct.

Tom

 

...man I thought the MIT guy was going to tell us about how Stokes law limiting viscosity drag coefficent puts a kibash on gasoline MPG derivative by the square root of car weight divided by effective surface area cubed. Turns out just too many Chrysler minivans is the reason (but guilty as charged).

...but I agree with gradual tax increase at the pump

 

I see what you're saying but in my mind I see no need for expending power to drive a turbo in the engine operating region that atmospheric pressure will perform the task with no "overhead" cost.

In other words as long as atmospheric pressure can provide sufficient cylinder fill levels right up the limit imposed by the maximum compression pressure of what good is "pressurization".

 

The MIT guy is an economist, not an engineer.

 

+1

Turbocharging allows you to downsize an engine to use a more appropriate size for the application, increasing overall efficiency. Cylinder deactivation and EGR can also downsize but unlike turbocharging there is an efficiency hit compared to a smaller engine.

West just doesn't seem to understand, and continues to spout non-sense. I would appreciate if he does not understand things to learn about them, instead of repeating the same wrong ideas.

The most efficient reciprocating engines are turbocharged though
Most powerful diesel engine in the world

Its huge and diesel and two stroke, so there is a hit getting small,light, gasoline and able to have inexpensive emissions control. Toyota says they have a gasoline di turbo engine running at 45% efficiency in the lab. I'm sure it is expensive for the prius though, and the prius engine is efficient across a broad range of rpm and power levels.

The principles is easy to understand. For certain initial compression, it is more efficient to do it outside of the engine. The air can also be cooled increasing maximum compression without detonation can be increased. If the power to pump this super charger and inter-cooler can come from the power in exhaust gases rotating a turbine. When this process is more efficient than pumping the air in the cylinder than the engine is more efficient. Boost delays and boost threshold have been improving, as well as a focus on fuel economy for powerfull cars and trucks.

 

In economics its simpler than that. If you want to use less of something, sometimes the most straightforward thing is to make it more expensive. This can cause suffering, and people can often not change quickly, which is why gradual is best. I know people with guzzlers that don't drive them very far.

 

Superchargers impose their own parasitic losses through the belt. An electric one will be even worse conversion losses. Granted, combined with a larger battery and regen braking, it could mean better overall system efficiency.

Without the super charger, an atkinson engine will have be enlarged to provide adequate power in a straight ICE vehicle. Advanced valve control will help, but it isn't available here.

For low boost turbos on a non-detuned engine see Volvo, or any backyard installed unit where the owner didn't want to run higher octane.

 

...

 

I don't know how this fits in with MIT research, but I know why my fuel economy is still low. I've been driving off-road in my Jeep, and when I filled it today my tank mileage was 14.5 mpg. It makes me respect my Prius all the more.

Tom

 

A supercharged engine hasn't been built that is more efficient by itself than a turbo charged one. But like the turbo, it can downsize an engine, so when used appropriately they can also make a car more efficient. In the past they have only been used to add power. VW has an interesting take, a small turbocharger and supercharger on its 1.4.L engine. The supercharger is only used when boost is needed and the turbo can not supply it, enabling a smaller engine with a more efficient turbo at high rpm, more power at low rpm. This does add cost and looks a lot like a hybrid turbo charger.

Yes, it would be beter on something like skyactiv with advanced valve control that can simulate atkinson or miller cycle and have the supercharger off when it is less efficient which is most of the time. You can do normal intake valve closing instead of the super charger. Or you could fit this engine with a turbo charger and make it more efficient

Yep.
The Future of the Internal-Combustion Engine - Feature - Car and Driver

I noticed west commented on my last comment and didn't understand anything.

 

One reason our fuel economy is not increasing is the amount of ethanol in our gas and the fact that the government wants to increase the amount of ethanol. Look at this graph and you will see that there is half the amount of energy in a gal of ethanol as there is in a gal of gasoline. It doesn't take much ethanol mixed with your gas to bring your mpg's way down.

 

What if you take the Prius A/C compressor's variable frequency inverter drive and motor and use it to drive, at a very wide range rotational rate (18,000 RPM bi-directionally), a 3:1 reduction geared input to a PSD type CVT, the opposite PSD input driven via the ICE. The PSD output shaft, RPM "sum" of the two inputs, driving a lightweight (carbon fiber) positive displacement SC.

No throttle plate, fully variable SC output would be used to control the ICE all the way from idle to FULL boost.

 

I'm shocked that I'm answering this again, but I'll try one more time. Because of expense we don't typically see turbo + super or hybid turbo chargers. When the turbo is not active it can have its waste gate open, no loss. When the turbo is active it uses the waste energy of the exhaust, so it is more efficient than the super charger. A super charger allows for better dp/dt, changes of power level, which is why vw is trying this. The way to find out efficiency is a bsfc chart, and car companies regularly make these and experiment with different turbos for the projected driving.

The vdub super charger can regulate it's boost. When a turbo charger creates too much boost for the engine part is vented through the waste gate. The higher the rpm, the more power the supercharger must take from the car for the same pressure. Turbo and supercharger can both regulate boost, but the supercharger requires extra energy. If you read the post you were replying to, one of the problems at higher rpms is lack of scavenging, some hot combustion gas is left in the engine. This is makes the engine both less powerful and less efficient. The exhaust and intake valves are opened, blowing fresh air through the engine, both cooling it and removing exhaust. With a turbo part of this energy is recaptured. With a supercharger it is too energy intensive and lower boost, cooled egr, and/or a richer less efficient mix is used.

I hoped that helped. In the vw 1.4 turbo they also do cylinder idling. The real question is will downsizing do any better than a 2.0L normally aspirated one with the latest tech like the skyactiv. I say its doubtful for the same NVH. But the payoff can be big when a 2L turbo replaces a 3 L NA engine. The prius 1.8L atkinson is like a 1.4L otto, so downsizing won't likely make it more efficient. This seems partially borne out by the prius c which the down sized to the 1.5L.