Turbo cars.... more efficient than NA on short trips?

[MAGN1T]

A turbocharged engine "recycles" exhaust gas energy that would otherwise be lost through the exhaust system on an NA engine. It uses this energy to produce MORE power during the next cycle.

That's another one that pops up all the time and isn't correct.

The piston has to do work to compress the exhaust to spin the turbine.

Efficiency and Economy are one and the same.

Although (As a general rule) turbo motors have a higher peak BSFC, the BSFC when producing say, 20hp required for cruising along, outside of their optimal rpm range, is significantly less flattering.

.

BSFC is usually quoted as pounds per hour per horsepower so a motor of higher BSFC is LESS efficient that a motor of lower BSFC.

Steve

[iOnic]

A turbocharged engine "recycles" exhaust gas energy that would otherwise be lost through the exhaust system on an NA engine. It uses this energy to produce MORE power during the next cycle.

That's another one that pops up all the time and isn't correct.

The piston has to do work to compress the exhaust to spin the turbine.

I wouldn't say the piston "compresses" the exhaust (it's not a sealed system) but you're right there is more resistance on the piston during the exhaust stroke in a turbocharged engine than there is in a naturally aspirated engine. That is the reason behind the reduced "economy" If you had read the next part of that post you would have seen this line.

Does this mean that the turbocharged engine is also therefore more economical? No, it does more work and therefore uses more fuel.

[Mr Revhead]

energy in vs energy out

that what it comes down to.

in the respect of this topic we are talking about fuel usage.
ie for this amount of fuel, this amount of mileage results.

i0nic you have the wrong end of the stick and are talking about power strokes etc.... ie the physical movement of the engine components which is not the same thing as the original poster is talking about.

[Mr Revhead]

A 2stroke engine uses more fuel yes.......but it also produces more power than an equivalent 4stroke which means there is less fuel being translated into waste (heat/unburnt fuel etc) so therefore the 2stroke is still more efficient than the equivalent 4stroke engine.

well.... only if the two strokes output is proportionally better in relation to its fuel use than the 4 stroke.

how much fuel would a two stroke use putting out 100hp compared to a four stroke that outputs 100hp?
trying to search google for decent figures but no one is listing hp vs fuel used for comparable engines.
would need ot know, engine capacity, power and fuel used.

[fivebob]

Piston ported two strokes are less efficient because they have a lower dynamic compression ratio.

Direct injected two strokes may well be more efficient because they spend less of their energy driving the engine between power strokes, but I can't find any BSFC data for direct injection two strokes to compare.

[RomanV]

Ionic, I think you are perhaps comparing VEHICLE economy to ENGINE efficiency.

The word 'economy' even has 'efficiency' as one of its definitions:

the efficient use of resources; "economy of effort"

But as an example of why turbo motors ARE less efficient when used over a wide rpm and load range,(As is emperically obvious by the gas milage)

Look at this chart, for a random motor:



Anywhere in the smallest 'island' is where this particular motor makes the most power, per unit of fuel used.

So at approximately 2500rpm, at what, 75%ish percent throttle, is where this motor does it's thing best.

Now look at the island with '500' written on it... In this rpm/load region is where TWICE as much fuel is required to make X amount of horsepower.

Now if you were planning on screaming around a racetrack with this motor, would it be more efficient than a motor which had its peak 'island' further up the rpm range, at a higher throttle %? Definitely not.

The % of time that daily driving puts each type of car in the low rpm, low throttle regions (ie. a lot of the time), and how the BSFC map correlates, is what makes a particular vehicle economical to run or not, when doing a specified amount of work.

This is why America has the milage tests, where they put a vehicle on a roller dyno and run it through a range of conditions to simulate highway and city driving, to determine a 'fair' MPG rating...

Manufacturers could cite the milage which you could get when under X amount of load at Y RPM, to make the numbers look good, even though it's not feasible to ever reproduce this kind of milage with normal driving.

So to say that turbo motors are more efficient (in all load/rpm ranges) is a blatant fallacy.
Emperical evidence definitely shows a distinct increase in the amount of fuel required to do a particular set of 'daily driving' type tasks in almost every instance.

It's just not possible for an engine to be more efficient (in terms of units of fuel required burnt to make X amount of power) and yet be less economical, in terms of fuel consumption.

[fivebob]

It's just not possible for an engine to be more efficient (in terms of units of fuel required burnt to make X amount of power) and yet be less economical, in terms of fuel consumption.

Yes, if you take it in isolation.

However real world situations tend to complicate things a bit. e.g. if the engine is matched to the wrong gearbox, and in normal use is outside it's best efficiency island, then it will be less economical than the same engine with different gearing. The overall efficiency of the engine remains the same, but it's economy is different because it's operated in different parts of it's efficiency curve.

[RomanV]

Agree 100%,

However point I am trying to make was to explain why "Turbo motors are more efficient" is generally untrue, when used in a real life scenario outside of the race track.

Bit of a sidetrack I guess, but a good non racing example of using turbo motors for efficiency that I dont think has yet been mentioned, are the Japanese Kei cars.

Instead of say a 1500cc or so NA engine, they use a 600cc turbo and/or supercharged motor, that you've gotta slog the guts out of to get anywhere.

But overall, it's more efficient and economical than a larger NA motor which could do the same thing, because revving it and putting it under load keeps it in its peak efficiency zone, in which it does deliver superior milage to some/all larger NA motor with less load and less rpms required to make the same hp.

I'd say the reason more 'economy' cars dont follow suit, is that ironically there's the stigma associated with turbos that they chew through a lot of gas, and are performance oriented.
Also increased production costs/complexity and reduced lifespan compared to a larger NA motor that isnt put under so much stress I guess.

There's the tax incentives to make this feasible in Japan, but I cant think of any similar examples elsewhere except for a European motor (VW golf?) which was a 1500cc twincharged, or turbo diesel or something?

I doubt there'd still be any 4Ks kicking around if they were 600cc turbo instead

[fivebob]

It's also because road cars are geared for passenger comfort and engine longevity that turbo cars are not more economical. Most turbo cars spend a large portion of their time off boost where their lower compression ratios make them less efficient. I suspect a turbo coupled with CVT may be more efficient.

Different story with trucks, trains and boats, there the turbo is put to good use, ethier by having a load to pull against, or by running at a constant speed/load, and it's superior efficiency can be demostrated.

[t0ms]

just to throw a spanner in the works

a lot of the two stroke quotes are largely redundant these days, especially in terms of outboard engines anyway

technology has solved the problem of "high fuel consumption" anyway

we rigged 6 boats, 3 with 115 hp 1.6 liter 4 cylinder 4 strokes and 3 with 115hp 1.5 liter 3 cylider dirtect injected two storkes.

given all were driven by a variety of different people in a variety of different ways, which do you think used LESS gas....

the two strokes