Turbo And Fuel Economy Discussion :P

[oldcorollas]

why is that?
in a diesel, it is due to lower pumping losses...
but in a petrol car, since the throttle is nearly closed, you are nto capabl eo futilising any boost or airflow produced by the turbo, so there is no difference in pumping losses on the intake side.

on the exhaust side, if anything, the higher exhaust manifold pressure (slight restriction of turbo) will increase pumping loss on the exhaust stroke.
so the only reason it could be more economical is due to decrease in loss of intake charge, due to slight backpressure in exhaust assisting in "effectively redicing cam overlap?

if not... how does it work?
i mean, how does the turbo burn fuel better when the throttle is closed and you are still using similar VE or air amount to NA?

[love ke70]

well the lower c/r of a FI motor should mean you can run a leaner ratio on the highway than a hard out NA motor. but whether you will be using more throttle with the lower c/r is another debate.

road trains use turbos because diesels love boost. you can go bigger and bigger with a diesel, but the best gain is to have it on boost all the time. you will hear then when you sit next to a big truck on the highway, as he puts his foot down, you will instantly hear the turbo spool. if he didnt have the turbo he would have no chance of keeping at speed.
petrol motors are different.
plus, u can run a diesel on boost and bugger all fuel with no harm done. which is different to a petrol motor.

maybe you should swap your petrol 4age for a diesel motor


ps. forced induction = midrange = for the win

[o_man_ra23]

My 18R-C sits a about 1/2 throttle on the highway (and is reasonable on fuel)... as it sits at 3000rpm. If I were to have a CT20 or CT26 hanging off the side, I could have it down to less than half throttle, and at 3000rpm the turbo would be spooling, which would increase the amount of air in the mix (less vacuum... even if not on boost). So by having a lower pressure differential across the throttle, there is less effective restriction, which means you will have less stroke loss.

[love ke70]

My 18R-C sits a about 1/2 throttle on the highway (and is reasonable on fuel)... as it sits at 3000rpm. If I were to have a CT20 or CT26 hanging off the side, I could have it down to less than half throttle, and at 3000rpm the turbo would be spooling, which would increase the amount of air in the mix (less vacuum... even if not on boost). So by having a lower pressure differential across the throttle, there is less effective restriction, which means you will have less stroke loss.

id like to see the practical outcome of this. i cant say i agree.
sorry.
the amount of throttle has little to do with restriction, but more the amount needed to keep the engine at those revs, and the car moving at the speed it is.
with a turbo, i do not believe you will use less throttle, or fuel.

[oldcorollas]

well the lower c/r of a FI motor should mean you can run a leaner ratio on the highway than a hard out NA motor. but whether you will be using more throttle with the lower c/r is another debate.

road trains use turbos because diesels love boost. you can go bigger and bigger with a diesel, but the best gain is to have it on boost all the time. you will hear then when you sit next to a big truck on the highway, as he puts his foot down, you will instantly hear the turbo spool. if he didnt have the turbo he would have no chance of keeping at speed.
petrol motors are different.
plus, u can run a diesel on boost and bugger all fuel with no harm done. which is different to a petrol motor.

compression ratio means buggerall when you have only 10% of the (possible) air going into the engine. i am not sure why you say that you can run leaner mixture at part throttle in a lower CR motor?

as you say, diesel is a completely different concept to petrol engine.
by using more air with less fuel, the heat from ignition heats more air, and you get more efficiency. the same would happen in a petrol motor if you could add say.. 2 bar of boost of nitrogen etc.
but there is no reason to compare petrol and diesel turbo engines when it comes to efficiency, as they are completely different mechanisms.

My 18R-C sits a about 1/2 throttle on the highway (and is reasonable on fuel)... as it sits at 3000rpm. If I were to have a CT20 or CT26 hanging off the side, I could have it down to less than half throttle, and at 3000rpm the turbo would be spooling, which would increase the amount of air in the mix (less vacuum... even if not on boost). So by having a lower pressure differential across the throttle, there is less effective restriction, which means you will have less stroke loss.

but.. your carby is twin throat no? so half throttle is actually less than half of full throttle (ie, the main is open half = less than half the area of main throat, but the secondary is fully closed).
if you had say, 300cc injectors (for 2l engine) running at half throttle.. ie, 40% duty cycle, thats 480mls fo fuel per minute. at 100km/hr this is 1.7km. giving a fuel economy of 29L/100km....

think of it this way. the power required to keep cam car moving at 100km/hr is the same, regardlesss of the engine.
if you were making "boost" pre throttlebody at 3000rpm on highway, then you have a BIGGER pressure differential across the throttle. ie, boost pre-TB, and "vacuum" post-TB.
if you had any boost after TB, then you are effectively at more throttle than flat to the floor with an NA car

basically... the manifold pressure after the TB, at highway speed, will be the same, regardless if it is an NA or a turbo car. if it is different, then the power output will be different, or, the efficiency is different. more air = more power = accelerate. less air = less power OR less losses but same power....


so.. given that the amount of air going into NA and turbo engine, after the TB, is basically the same on a turbo and NA car at highway speed... why woudl the turbo car be mroe efficient?
if it is making boost, that means there is pressure pre-TB, and to get pressure pre-TB, you MUST be introducing restriction into the exhaust, to extract power to spin the turbine and compressor... and that restriction MUST increase exhaust manifold pressure.... see what i am getting at?

[o_man_ra23]

but.. your carby is twin throat no?

Would be if the secondary worked

Now lets go back and think about this again... when I say power in this, I mean torque... most people dont truly know what torque is and why its better than power though.

We aim for a stoichiometric burn right?? And why cant we do this... because petrol is too volatile and combusts too quickly. Without resorting to direct injection, we are stuck running the car rich and having unburnt fuel out the exhaust. Diesel is already capable of a much more controlled burn, giving better efficiency as you can run closer to stoichiometric AFRs. We are pretty well stuffed on that, its a fact of life and we use timing and mixture adjustment to counter it.

So how can we improve our air efficiency... ie get more bang out of the same amount of air?? We increase compression is how. A higher compression ratio will cause the fuel to become even more volatile and burn harder. The problem with this is detonation, which we control by timing. In this case, we are sucking the same amount of air into the motor as a lower compression engine, but we are getting more out of it. As we are getting more out of it, we dont need as much fuel to make the same power at a given load... or we can get more power with the same fuel. Downside is that we are using the same energy to draw air in, and more energy to compress it to ignition. So how do we reduce these losses??

Turbo... exhaust gasses go in, witchcraft happens, and you go faster (shamelessly taken from Jeremy Clarkson). The exhaust gasses are normally pushed out the exhaust and their restriction is a cat converter (to reduce emissions) and a muffler (to reduce noise) as well as a pipe (to move the gasses beyond the cabin). We cant go too big a pipe on the street due to noise regulations... same with the muffler. Cat converter we are stuffed and have to have. Adding a turbo into this system only adds to the restriction (as mentioned by OC) and becomes a problem. BUT... due to the fantastic noise reducing properties of a turbo's turbine wheel, we no longer need such a restrictive exhaust. In many cases you can remove the need for a muffler all together!! See Brett_Celicacoupe's 100% legally engineered mufflerless 1G-GTE RA40... now sold so you will need to look at the writeup. That car with a 2 1/4"?? exhaust, XF foulcan CAT and no muffler was by far and away quieter than my 18RG RA23 with 2" exhaust and sport muffler (and also quieter than my 18RC RA40 which uses the same exhaust donated from the RA23).

SO... yes the turbo does add restriction, but it also allows you to take restriction away, thus negating that loss. So if we set the 2 motors up such that the dynamic compression when cruising at 3000rpm is even, we then have equal stroke losses on compression and equal stroke losses on exhaust. BUT... seeing as we now have a compressor wheel pushing gasses at the pistons, we then dont have as much suction loss through the throttle body, as the pressure in the intake will be higher than atmosphere (same theory as having a forward facing scoop ramming into your intake), so it doesnt take as much effort to get the same gasses in (the turbo motor can be smaller to get the same gasses in). So... we now have LESS losses in the turbo engine to get the same amount of air to the same compression at the same engine speed under the same load. Which means we will need the same amount of fuel to produce the same amount of power at the flywheel (assuming all drivetrain losses are equal, this means the same amount of fuel to put the same amount of power to the wheels), but we need LESS fuel to counter the losses.

Therefore... a turbo is more economical on the highway than a NA car. (and Superchargers have greater losses - mechanical belt driven, so they are less economical)

Make sense??

[oldcorollas]

sorry.. no...
i am fairly sure i am familiar with torque.... however, to remain at a certain speed, regardless of the engine type or rpm or gearbox or diff gearing, saying you need XX power to remain at a certain km/hr is actually more relevant

modern cars can and do cruise at leaner than stoich mixtures.

look at the manifold pressure during cruise.. maybe.. 40-50kpa absolute? 'depends on cam and whatever). the amount of air going in is not so much, velocity within intake system is not so high, and the pressure differential between atmospheric and the engine intake vacuum is enough such that the throttle body is the major (and pretty much only) restriction.

if you have a turbo making any boost, that "boost" is just a measure of the pressure between the throttle body and the turbo... NOT going into the engine.
having a higher pressure differential across the throttle body does make it easier for the air to pas the throttle body, so it means you can have lower throttle input.. ie throttle body is closed more..

BUT, the amount of air going in is the same, and so the amount of fuel used is same.

however this assumes two things.
the cam of the NA motor does not have overlap such that at the cruising rpm, you are losing fresh charge out thru exhaust valve.
the exhaust backpressure from the turbo (which MUST be present if you are makign any boost) is not sufficient to increase pumping losses on blowdown.

in practice, what i think happens, is that there is little or no boost in the manifold, however with no turbo restriction in exhaust, a small amount of fresh charge can be lost thru the exhaust valve during overlap. with the turbo restriction, the loss of fresh charge is reduced

this means less air/fuel is lost without being burnt.
this means that less air/fuel need to go in for same bang.
this means you actually have LOWER intake manifold pressure (post TB) in turbo motor, and use less fuel...

but is it actually true?

most of the time, driving my car, it is not making boost at all, and is almost always in "vacuum", unless accelerating reasonably vigorously....
there is DEFINITELY no boost being measured at highway speeds...

[oldcorollas]

to back that up.. from the caldina catalog i got recently, there are a number of engines available, in almost the same weight/drag car.

3sgte
3sge (vvti)
3sfe
3cte (diesel turbo)

if you travel constantly at 60km/hr, these are the official km/L figures from toyota (with car weight in brackets)

3sgte (1440kgmanual/1470kgauto) 16.4km/L man, 15.3km/L auto
3sge (1410kg auto) 18.2km/L auto
3sfe (1340man, 1360auto) 20.7km/L man, 20km/L auto
3cte (1410 auto) 21.5km/L auto

diesel is best economy at 21.5.. (3sfe is a bit lighter, but at constant speed it shouldn7t matter so much), then 3sfe at 20km, then 3sge at 18km, then 3sgte at about 16km/L

the diesel is 2.2L and same torque as 3sge, more torque than 3sfe. it has 94HP vs 260HP for 3sgte.....

3sgte uses most fuel, for roughly same weight vehicle, at same speed......

[o_man_ra23]

3S-GE is VVTi, so that in itself is a whole nother ball game. 3S-FE has less overlap and 3C-TE is Diesel so again, a whole nother ball game. Also the FE is 100kgs lighter, which as you said shouldnt matter so much at constant speed, but it does add more friction losses to the bearings. Also, is the 3S-FE 2WD?? as that alone would make a big difference in friction losses.

I also never said you would be making boost. I said the turbine would be pushing air in, thereby reducing the vacuum. Boost is normally measured relative to atmosphere... not intake plenum. I would really doubt you would be making boost when cruising at highway speeds... unless you had a tiny turbo.

[oldcorollas]

they are all 4WD. the weight difference is not that much...

find a car which has both turbo and non-turbo variants where the turbo has better fuel economy

i'm not saying it cannot be true, but your reasoning (to me) seems not correct.
sure the turbo is pushing air, but it can7t push air unless it restricts somewhere else..

see, from the air filter to the throttle body is basically no restriction, at the intake velocities we are talking about...
if the turbo is pushing air in there or not, it will make basically no difference, i am sure if you measure the pre-TB pressure, relative to atmospheric, on both a turbo and non-turbo car at highway speed, they will both have atmospheric pressure there... ie, no difference on the intake side.

i still maintain that it is the turbos restriction of the exhaust that will make any difference (if any) to improve fuel efficiency..

i think people usually just confuse diesel turbo fuel efficiency with petrol turbo fuel efficiency... when they are actually completely different

as for a difference of 100kg making a difference to the bearings....how hot do bearings get? i cannot imagine them needing kw's or even hundreds of watts of power to turn them, even with the extra weight. if that was the case, bearings would overheat and fail in hours,... not thousands of hours...

[Bored?]

If you factor in tyres:

W=F*d, F=Crr*Nf, W=Crr*Nf*d
At 60kph, d for 1second = 16.6 0 meters. Assuming a Crr of 0.02[1]
W=0.02*16.6*9.8*m = 3.25m

So for a 1400kg car that's around 4.5kw of rolling resistance at 60kph.
Adding 50kg would increase resistance by only 162 watts.

[1] http://en.wikipedia.org/wiki/Rolling_friction

[cambelt]

A 7M-GTE uses more fuel than a 7M-GE
just like the NA JZ uses less fuel than the GTE models.
And again with the T motors.
Dave

[o_man_ra23]

Something that slipped my mind before... Toyota turbo engines are tuned REALLY rich from factory (often with AFR's of around 10), so that may have a lot to do with the economy difference shown with factory figures.

I dont think I can prove my point, but I dont think anyone can disprove it either. Seems to be a stalemate unless somebody builds a test engine.

[Sciflyer]

My n/a 1UZ Soarer runs sub 11:1 afr's at WOT and i can still get just over 10 L/100km without even trying...

Ok so thats anecdotal, but still...

From the turbo bible, Corky Bells' Maximum Boost

[oldcorollas]

Something that slipped my mind before... Toyota turbo engines are tuned REALLY rich from factory (often with AFR's of around 10), so that may have a lot to do with the economy difference shown with factory figures.

I dont think I can prove my point, but I dont think anyone can disprove it either. Seems to be a stalemate unless somebody builds a test engine.

rich under boost for safety.. but they have oxygen sensors for a reason

but still have worse cruising economy
the constant 60km/hr test is about as close a factory test as you will get for comparing engines for economy.... the other test methods include accell and decel, so they are not a measure of cruise efficiency...
in those tests also, the turbos are worse...