Adventures in Hypermiling - Increasing MPGs

[DanceswithUnix]

So, based on all of that information, are you telling me that if I removed the turbo from my car somehow, I would return better MPG figures for the same speed of travel? That doesn't make sense to me at all.

Sort of. The small engine gives you small engine economy at low revs. The problem is at high revs you would be better off with a bigger engine. The point of a turbo is it is *almost* like having a choice of two engines depending on whether it is spun up or not, but it is a compromise not a straight win as driving any compressor isn't free.

[kalniel]

And yes, once you're at speed, removing the turbo might well be better, assuming enough power to overcome the air resistance. However, getting there is more fun with a turbo

[Hoonigan]

Now, without going into the massive amounts of detail involved, I can say, without doubt that having a turbo on a diesel engine increases its efficiency considerably at motorway cruising speeds. I've read plenty and watched a couple of videos tonight.

Considering the turbo is just spinning away, using what energy would've been wasted anyway. The small amount of power that it takes from the exhaust phase of each compression is more than made up for in the increased pressure inside the engine, ensuring all fuel is burnt and less is wasted. Modern ECUs are designed to work this way.

If you genuinely believe that a turbo is designed purely to increase power and not motorway efficiency then I'd appreciate some actual proof or evidence to support that.

[joshwa]

Update:

Over the last 592miles, I got 66.78mpg, this was sticking to a max of 60mph, switching off at lights, and half a redex in the tank.

* This may not be a result of the Redex, my driving / test is not under scientific conditions, and my driving patterns may have changed. But it also may be due to Redex.

[Macman]

The car is capable of about 40mpg, but I get bored and distracted which is dangerous so I actually get about 33mpg. I only get 40mpg on motorways.

Shouldn't be on the road if that's the case. Your not only a danger to yourself but others!

[badass]

The dirty secret of modern turbo petrolengines is that they are only economical when you *don't* use the turbo.

FTFY

Diesel economy is fine on boost. In fact pretty much all modern diesel engines are more or less permanently boosting. My one that doesn't feel like it even starts boosting until at least 1600 RPM is actually hitting 8 PSI long before that. One advantage of having no throttle restriction.

As a slight extra bit of info, manufacturers are trying to improve on boost economy and are making some progress. It's one reason for integrated water cooled exhaust manifolds being built into cylinder heads. Less full throttle enrichment needed. Still some needed however so there's some way to go still.

[badass]

Recently I've been doing a lot more miles in my VW Polo 1.4TDI (75ps, 2004),
Stick to max 70mph: 51-54mpg
Stick to max 65mph: 57-59.72mpg
Stick to max 60mph: 63.91mpg (I also switch off the engine at long stops at red lights)
Stick to max 57.5mph: 64.05mpg (expected a bigger improvement)
Back to 60-65mph: 59.72mpg

And here is why the tiny engined diesel cars are such a complete waste of time, effort and money. Not only did they fill our cities with more pollution, get clogged DPF's but they are also worse for economy for motorway driving, something diesel engines have traditionally excelled at.

I have a 2006 168 BHP Golf TDI. The engine is not very efficient and the car is heavier than a polo as well as having more aerodynamic drag (a higher CdA) It also has a DPF and when in regen mode it uses more fuel as that's how regen works.

My figures:

Stick to max 70mph: 51-55mpg
Stick to max 65mph: 57-60 mpg
Stick to max 60mph: 64-66mpg (I don't bother to switch off the engine at long stops at red lights)
Stick to max 57.5mph: 64.05mpg (expected a bigger improvement)
Sit behind a truck for 100 miles to see what effect wind resistance has. From a cold start over 100 Miles in the winter at sub 7 degree temps: 70 MPG. Unbelievably boring.
Economy for the next 100 miles after that: 44 MPG

My figures may seem high but it's not unusual for me to not use the brakes at all in a 100 mile period. A single hard braking from 70 to 40 MPH and then gently back to 70 MPH easily cost 1 MPG over 100 miles.

The reason the small engined diesel cars are the same or worse on fuel on the motorway? Revs. Diesels lose more of their relative efficiency due to the higher compression ratio. It's why they have more engine braking than equivalent petrol engines despite taller gearing.

[Zak33]

Now, without going into the massive amounts of detail involved, I can say, without doubt that having a turbo on a diesel engine increases its efficiency considerably at motorway cruising speeds. I've read plenty and watched a couple of videos tonight.

Considering the turbo is just spinning away, using what energy would've been wasted anyway. The small amount of power that it takes from the exhaust phase of each compression is more than made up for in the increased pressure inside the engine, ensuring all fuel is burnt and less is wasted. Modern ECUs are designed to work this way.

If you genuinely believe that a turbo is designed purely to increase power and not motorway efficiency then I'd appreciate some actual proof or evidence to support that.

OK.. i'll explain:

"turbo is just spinning away, using what energy would've been wasted anyway" is not what's happening
There is pressure trying to slow the turbo sand the engine. Both pressure blocking the exhaust (turbo exchaust vains) and from compressing the air going back into the inlet. You're wise enough to know there is no gain for nothing. There's always a cost.

Exhaust gas needs to come out with no resistance, right? so it doesnt back up in the system and cause the pistons tonot push the next exhaust gas pulse out. Put a banana up your exhaust and see how long the car runs .... it won't. Exhaust is blocked.
A turno works by gettingi in the way of that exhaust gas to turn a propellor. In doing so its slowing the exhuast gas. That's what turbo lag is.
And it's not just spinning free.. it's trying to suck more air in with the other end of the turbo... and sucking air in faster than it wants.. is hard work . Try breathing through a straw while jogging.

But the COMBINED EFFECT is to force more air into a small engine and get more power out than it would normaly create. But it's not free power!

the reason turbo's on a modern car are so popular is that at tick over in traffic... it uses the tiny amount of fuel of the small capacity engine and its a very light engine so the car is lighter. As speed goes up it IS capable of pulling a large car that a small non turbo engine cant do properly

BUT please don't think it's some 100% efficient system cos it aint. A larger nomrally aspirated engine at motorway speeds would be more efficient. But not up to that point.

Cars are about balance and the balance is currently between environment, fuel efficiency (not the same thing) and cost

[Zak33]

As an aside... turbo's cause other issues. They eat engine oil way faster. The turbo needs oil and it's HOT AS HELL and it destroys oil faster than a NA engine.
They therefore need VERY consistent servicing and skipping oil changes is a v bad idea. They need cooling off more after a quick run as the turbo bearings need to cool. Sadly the car brands are also making quite low quality turbo housings and as many of them are in the exhaust manifold, when the turbo fals or wears ... you need to change the whole manifold too. Many of the new breed are 3 cylinder engines and while it's a good size and very light, it's naturally not balanced. I hate 3 pots. Personal preference over engineering? Maybe but it is looks right it normally is - still applies. Turbo's are harder to use in snow and ice on hills. When you really need torque, but in gentle amounts a turbo is a pain in the bum. As you pull away you need a few revs.. if wheels spin up either the traction control kicks in and cuts the power and the boost drops or the bost spins up the tyres causing more spin and then more boost. I hate small turbo's in snow!

[Hoonigan]

But, the additional fuel required to spin the turbo is more than made up for by the economy gains, which are made possible by burning the same amount of fuel more efficiently. This is through the addition of more oxygen than ambient pressure allows.

I don't see how there can be any argument against this.

Turbos improve fuel efficiency and fuel economy.

[DanceswithUnix]

Shouldn't be on the road if that's the case. Your not only a danger to yourself but others!

OK, badly phrased so that did sound like I am some sort of frothing madmad. I'm not, it's fine
I do find that a small amount of driving aggression is better for me though. It is just a little, for example I hate mixing with the commuter traffic coming out of London. I'm like an old granny compared to them.

FTFY

You are right that petrol turbos are way more sensitive to boost. But diesel cars were around before turbos got strapped on them, and the few I came across had awesome fuel economy. What they didn't have was performance.

[DanceswithUnix]

But, the additional fuel required to spin the turbo is more than made up for by the economy gains, which are made possible by burning the same amount of fuel more efficiently. This is through the addition of more oxygen than ambient pressure allows.

I don't see how there can be any argument against this.

Turbos improve fuel efficiency and fuel economy.

Fuel must be burnt with the correct amount of oxygen. Too little and you get black smoke and poor economy, too much and you get NOX emissions. That's why when you are driving down the motorway on part throttle your diesel engine not only has the turbo deactivated but you probably have an EGR valve directing exhaust gas back into the intake because when you need so little power you don't even need the amount of oxygen in atmospheric pressure. That short circuits the turbo, it would have to spin like the clappers to overcome the bypass hole (which is why diesels with a stuck open EGR are really hard to drive, I've been there).

[PC-LAD]

Fuel must be burnt with the correct amount of oxygen.

Yes, My dad owned a 2004 3.5ltr Nissan X-Trail. It was a beast, so much so that the last time it was driven we were in a rush and put his foot down on a 100,000 mile turbo. No regular oil changes, then we got a whur from the turbo spooling (it didn't stop) then we had our engine runaway. at 80mph he put his hazards on spent a good 5 min trying to get out of gear as we was over the highest revs and overheating fast. he managed to stall it. 10,000 miles later on the way to work, the engine blew. (p.s it was a renault engine, frenchie engineering not always the best)

[Butcher]

Fuel must be burnt with the correct amount of oxygen. Too little and you get black smoke and poor economy, too much and you get NOX emissions. That's why when you are driving down the motorway on part throttle your diesel engine not only has the turbo deactivated but you probably have an EGR valve directing exhaust gas back into the intake because when you need so little power you don't even need the amount of oxygen in atmospheric pressure. That short circuits the turbo, it would have to spin like the clappers to overcome the bypass hole (which is why diesels with a stuck open EGR are really hard to drive, I've been there).

Exactly this. You don't need a turbo to burn lean at low load, you can just inject less fuel with a wide open throttle (which is effectively what diesels do). That gives you both lean burn and no turbo restriction on the exhaust which should be even better.
In the real world though two things happen. Or rather one thing happens, with two bad side effects. Lean burn is hot. Really hot. So hot that it tends to melt engine components. Also, because it's really hot, you get more NOx. EGR is primarily a way of cooling combustion temperatures down by reducing oxygen in the mix. Petrol engines tend to just restrict airflow instead to get a similar effect.

As an aside, the best way to get better fuel economy on the motorway is to buy a smaller or more aerodynamic car (or both). Drag loss is the largest factor once you're above about 40-50 mph.
Anecdotally, I get significantly better fuel economy on my NA 2 litre petrol than I did on a similar power 2 litre turbo petrol. A lot of that is because this car has about 25% lower aero drag.

[Zak33]

But, the additional fuel required to spin the turbo is more than made up for by the economy gains, which are made possible by burning the same amount of fuel more efficiently. This is through the addition of more oxygen than ambient pressure allows.

I don't see how there can be any argument against this.

Turbos improve fuel efficiency and fuel economy.

Firstly, fuel and air mix in a petrol car must be in perfect union. Only in a diesel can you run excess air. As boost pressure rises, in a petrol car, petrol injection rises too. It has to to increase power.

Now - ignore the fuel for a moment and lets look at how the turbo boosts and how is gets it's spin speed to make that small engine work harder/

What makes the turbo spin? Exhaust gas, right?
But to do so it has to get in the way of the exhaust gas.. its a propeller, bocking the exhaust pipe. So it blocks the exhaust and in turn slows the engine while is catches the exchaust gas and converts it to spinning motion. Don't forget this is all happening while the car drives. In theory you want exhaust gas to not be blocked from leaving otherwise as the piston travels up pushing the waste gas out, it slows down, reducing power.

The other end of the turbo shaft SUCKS air from the air filter (quite hard work at ths speeds it needs to suck in at) and BLOWS it under pressure into the cylinder. But to push air faster than it's natural speed involves compressing it. and to do so it takes immense effort.. and that effort COMES from the exhust system . We must not forget that it's not just pushing air in.. ir has to pull in in through the inlet system of pipes.

What I'm getting too is this - turbo's give excellent power increases on smaller engines without a large increase in engine weight. That's all awesome.
But the turbo itself is not 100% efficient.

That's why turbo lag exists.. spooling up the turbo to a speed where it's blowing more than it's causing drag on the exhaust takes time and it happens everytime you accelerate.

Take that a step further - the turbo is COMPRESSING air beyond it's natural atmospheric pressure BEFORE the inlet valves shut and the piston squeezes it EVEN MORE. The piston now has to drive up squashing air /fuel thats a higher pressure than the last time it did it split second ago. And there are 3 or 4 or 6 of them alternating that. SO one cylinder is trying it's best to push DOWN on the piston with a big BANG and one of the others is desperately trying to squeeze exhasut gas out through valves and past a turbo thats catching all thaty pressure and which is also trying to suck fresh air in and squeeze it hard.... it's all a blockage.

that all takes effort. So you DO get more power than the equivalent sized engine could create.. but it's not a perfect 100% improvements. Not even close.

Now.. imagine how much air a nornally aspirated engine needs to suck in at 4000rpm or 5000 or even 6000 rpm. It's immense and it starts to struggle.. there's a vacuum in the entire intake system all the time and it sucks in hundreds of litres per second through quite small tubes.

Now do the same but with a turbo.. it's having to spin SO FAST and suck in EVEN MORE air than it's N/A sister... AND squeeze it hard enough into the cylinder that it's forcing the pressures up and up ...
Engines at high revs still suck air in as the valves are closing, the cylinder isn't as full as it is at tick over... and the exhaust valaves at high revs open before the detonation is complete.

the engine dev team have to choose a cam profile to open anjd close valaves to match a torque style that they're aiming for....and match that to the turbo boost pressures at all the rev ranges and all the engine loads.
But it isn't spinning a free turning, unburdened shaft -The other end of the turbo is also a propellor, to push air INTO the engine. So it has a load

none of this is free of charge on efficiency.

this is a circular, clyclic series of events with other cylinders in the same engine doing the same tihngs but out of phase.

One piston is moving upward compressing a load of high pressure air and fuel, squeezing it so fast tight you'd never manage it yourself as a human, and it takes effort.
One piston is moving down drawing in air through a series of pipes and an air filter, and is being speeded upby a turbo
One piston is moving up pushing exhaust gases out through a tiny pair of holes only to find a bloody great propellor is blocking its way to use this exhaust gas pressure to turn the turbo we see in the previous sentence
One piston is moving down under the fresh explosion of all that sweet sweet air and fuel... but it's doing so while not just moving that 1 ton car forward.. but doing the three things above all at the same time

Turbo's are astounding things. But they are not free of charge

[Ttaskmaster]

Dunno if anyone watches Car Throttle, but they just did a vid on getting an RX-8 to go London to Edinburgh on a single tank... !