Are combustion engines as efficient as they can be?

[T0M]

I am curious as to know whether the latest crop of modern super frugal diesel engines such as that in the VW Polo Bluemotion for example, with a combined figure of 80.9mpg are likely to be about as economical as the modern combustion engine can get?

Seems over the last 5-10 years or so the combustion engine has been really squeezed in order to increase efficiency and reduce emissions. Have we taken the biggest step in terms of effiicency out of an engine as feasibly be?

For my dissertation I have been doing some calculations based on CO2 emission output from cradle to grave between the Polo Bluemotion and Nissan Leaf. Whilst the Leaf does work out a fair bit cleaner, the only reason it is comparable at all is due to the high % fuel mix of coal and gas in UK power station. There is however greater potential for this CO2 figure to lower significantly with greater use of renewable energy and nuclear power (& commitment to EU renewable energy directive. However, the combustion engine is stuck with using oil, so the greater efficiency can only come from a higher miles / gallon ratio.

Whatcha rekon?

P.S - I'm not one of these eco warriors either, just researching for dissertation & general interest purposes. I'll take a VXR8 Bathurst Edition any day of the week

[kiethton]

its my understanding that combustion engines could be far more efficient (in terms of mpg) if they didn't have to worry about emissions, i may be wrong though

[maniac0796]

I think people are too fixed on MPG and CO2 figures to think about the larger effect cars have on the planet.

How much damage does it do to the earth mining for all those lovely precious metals in the cat converter, or the metals for your rechargeable batterys. Or the electricity used to recharge those batterys.

Then there's other stuff. Like, apparently, the biggest container ships out there at sea use more oil in a day then you could in a lifetime. And how NiCad batterys, the Nickel is mined in the USA, causing loads of pollution, shipped across the globe to be refined, then shipped again to be made into batterys, then shipped to car factorys, and then those cars are shipped to dealerships.

Also, what do you mean by efficiency? If I was an engine design, I'd pride myself in getting more HP and torque out of an engine with lower capacity rather than making a laborotory MPG figure.

Edited by maniac0796 on Sunday 1st May 18:26

[T0M]

kiethton said:

its my understanding that combustion engines could be far more efficient (in terms of mpg) if they didn't have to worry about emissions, i may be wrong though

Interesting. Not heard that before, shall research it. Also seems crazy that most engines are only 18-20% efficient due to heat loss, sound, friction etc.

[T0M]

maniac0796 said:

I think people are too fixed on MPG and CO2 figures to think about the larger effect cars have on the planet.

How much damage does it do to the earth mining for all those lovely precious metals in the cat converter, or the metals for your rechargeable batterys. Or the electricity used to recharge those batterys.

Then there's other stuff. Like, apparently, the biggest container ships out there at sea use more oil in a day then you could in a lifetime. And how NiCad batterys, the Nickel is mined in the USA, causing loads of pollution, shipped across the globe to be refined, then shipped again to be made into batterys, then shipped to car factorys, and then those cars are shipped to dealerships.

Agreed. It wasn't my plan to turn this into an electric vs petrol debate though as there are many of those already in full swing on here

[lozzzzzz]

Efficency and MPG aer two very different things, linked in places. For example a diesel is more effient than a petrol, and the diesel gets better MPG, but thats about as far as it goes.

Efficiency is how much energy you're getting out compared with the chemical energy you're putting in. So a race tuned engine is normally quite efficient, but will geice low MPG. A Diesel driven at low rpm will give good mpg, but will be opperating away from its most efficent region.
I'll not get too carried away, I love this subject.

All production engines could get significantly higher MPG if the emmisions crap was removed. The compression ratio is seriously limited to reduce NOx emissions, the exhaust is restrictive for noise reasons and strangled with the CAT.

The development of lean burn engines (the modern GDI, FSI etc) was seriouly delayed in the 90's because legislation dictated that CATs would be fitted, and CATs require stokeometric burn (or they did then). So they are only now being introduce and making a massive difference.

Thats what I remember for Uni a couple of years back.

[Willy Nilly]

lozzzzzz said:

Efficency and MPG aer two very different things, linked in places. For example a diesel is more effient than a petrol, and the diesel gets better MPG, but thats about as far as it goes.

Efficiency is how much energy you're getting out compared with the chemical energy you're putting in. So a race tuned engine is normally quite efficient, but will geice low MPG. A Diesel driven at low rpm will give good mpg, but will be opperating away from its most efficent region.
I'll not get too carried away, I love this subject.

All production engines could get significantly higher MPG if the emmisions crap was removed. The compression ratio is seriously limited to reduce NOx emissions, the exhaust is restrictive for noise reasons and strangled with the CAT.

The development of lean burn engines (the modern GDI, FSI etc) was seriouly delayed in the 90's because legislation dictated that CATs would be fitted, and CATs require stokeometric burn (or they did then). So they are only now being introduce and making a massive difference.

Thats what I remember for Uni a couple of years back.

Is there much difference in the specific fuel consumption of diesel and petrol engines? Diesel is denser than petrol, which is why (I think) you get more MPG's, but do you get more grammes per kilowatt hour?

[Steve H]

T0M said:

kiethton said:

its my understanding that combustion engines could be far more efficient (in terms of mpg) if they didn't have to worry about emissions, i may be wrong though

Interesting. Not heard that before, shall research it. Also seems crazy that most engines are only 18-20% efficient due to heat loss, sound, friction etc.

Very true.

Engines could run a lot leaner (thinking more petrol than diesel here but the same may apply) by using advances in cylinder head design, direct injection etc but typically all these things result in a high compression, high combustion temperature engine which would tend towards producing excessive oxides of nitrogen. CO2 isn't the only nasty emission that internal combustion can come up with.

[ZeeTacoe]

maniac0796 said:

I think people are too fixed on MPG and CO2 figures to think about the larger effect cars have on the planet.

How much damage does it do to the earth mining for all those lovely precious metals in the cat converter, or the metals for your rechargeable batteries. Or the electricity used to recharge those batteries.

Then there's other stuff. Like, apparently, the biggest container ships out there at sea use more oil in a day then you could in a lifetime. And how NiCad batterys, the Nickel is mined in the USA, causing loads of pollution, shipped across the globe to be refined, then shipped again to be made into batterys, then shipped to car factorys, and then those cars are shipped to dealerships.

on the other hand a container ship is moving hundreds of thousands of tonnes of stuff about for that relatively small amount of fuel. 6 tonnes an hour for 150,000 tonnes of cargo ain't that bad.

Even better I've just done a quick calc based on how long I've had my car. 53,000 miles at roughly 43mpg is about 1232 gallons or 5595 litres. give or take 5.6 tonnes. So about 140years. But that is a figure based on running the engine flat out for 24hours and for a commercial ship extremely very unlikely. You could halve that by slowing the boat down and losing loads of drag.The Emma Maersk has a fairly fuel efficient engine which has a BSFC of 0.260lb/hp/hour way better than any car.
http://www.robse.dk/pages/Emma/EmmaFami.asp

maniac0796 said:

Also, what do you mean by efficiency? If I was an engine design, I'd pride myself in getting more HP and torque out of an engine with lower capacity rather than making a laborotory MPG figure.

hp/l is overrated. it exists only to cheer up owners of highly boosted cars during engine rebuilds.

[thinfourth2]

I do hope you are comparing like for like as many folk look at the headline figure of CO2 car for a car and completely ignore the fact the fuel has to get out of an oil well across the planet get refined and then delivered to the petrol station.

They then look at a electric car and base the CO2 on the fact the fuel has to get out of an oil well across the planet get refined and then delivered to the petrol station.


But when looking at efficiency look at how much damage emission controls make to efficiency as injecting exhaust gases back into the intake ain't good for efficiency.

[lozzzzzz]

Willy Nilly said:

Is there much difference in the specific fuel consumption of diesel and petrol engines? Diesel is denser than petrol, which is why (I think) you get more MPG's, but do you get more grammes per kilowatt hour?

The specific fuel comsumption is higher for pertols, buts thats just another way of saying that they use more fuel for hte same power output.

There are a number of reasons for diesels being more efiecnt than petrols

1. They have much higher compression ratios (with the exception of direct inject petrols). This means they have much higher expansion ratios so more of the heat in the hot gases after combustion is converted into work (this is why diesels have cooler exhaust gases).

2. Diesels have no throttle valve (power is controled by fueling only). So there are no pumping losses, i.e. in a pertrol the engine is using energy pumping the air past a nearly closed throttle valve (except whem you floor it).

3. This is complicatoed one. In a petrol engine air and fuel go in and fill the cylinder, the whole charge burns (simple so far), the burnt gases near the cyulinder wall lose lots of heat to the cylinder wall, this is energy lost that can no longer be turned into useful work.
In a diesel, the cylinder fills with just fresh air, near the end of compression the fuel startes to be injected. On light and medium load only a small amount of diesel is injected and the little flame thats created doesn't reach the cylinder wall (not as much as a petrol at least).
So picture a flame ball in the certre of the cylinder with a blanket of fresh air around it, this insulated the hot burnt gases, so less energy is lost to the cylinder wall. Its not this black and white in the cylinder, but this effect is significant.
This is why diesels take so long to warm up when idling on a cold marning.

4. Diesel has a little more energy per kg than petrol, this helps too.

5. Diesels tend to run at (much) lower RPM, so there are far less losses due to friction. When a petrol revs high, a massive amount of energy is used up as heat due to friction.


Its interesting when looking at the above it becomes apparent that the most ecconomical way to drive a petrol engine is with you're foot all the way down (so there are no pumping losses) and keep the reves really low (to keep friction to a minimum). I managed 53mpg (I kid you not) from my MR2 mk1 driving like this, it was boring though.

[Engineer1]

Don't forget that currently the vast majority of engines use linear motion of pistons to produce rotary motion, so in theory a wankle could be more efficient still.

[anonymous-user]

The principle efficiency limit to a production IC engine is simple a cost one. We could recover almost all the wasted heat and improve efficiency massively, however, with current technology and fuel prices, it is simply not worth it......


(google "BMW steam car" for more on waste heat recovery!)

[Asterix]

I also thought block thickness came into play for thermal efficiency as well for diesels - having to be thicker and stronger for the increased comp ratios.

[tercelgold]

It's not really worth it with high mpg if the car costs more to buy or maintain.

Saving say £344.75 a year between 80mpg and 150mpg, are you really going to make that back
in the car's lifetime ? DPF and dual mass flywheels are bad enough.

10K miles, £1.30 a liter

60 mpg £984.99
80mpg £738.74
150mpg £393.99
400mpg £147.75

[lozzzzzz]

I'm with you on the dual mass flywheels, they SUCK

The BMW steamer looks awesome, I've never seen that. Another great idea like this is Bruce Crowers 6 stroke engine. A friend of mine experimented biulding one for his dissertation.

The block cylinder wall is thicker on a diesel, and the pistons and rods are rienforced, making everything a little heavier and slower.

I believe John Deer experimented with a wankle Diesel. The major drawback of a wankle is the increased surface area. There are a lot of benifits, but it'll never quite be as good as a cylindrical piston engine because for the same volume of charge there is lot more combustion chamber surface area for heat to excape through.

Sorry for my bad bad spelling.

[maniac0796]

ZeeTacoe said:

maniac0796 said:

I think people are too fixed on MPG and CO2 figures to think about the larger effect cars have on the planet.

How much damage does it do to the earth mining for all those lovely precious metals in the cat converter, or the metals for your rechargeable batteries. Or the electricity used to recharge those batteries.

Then there's other stuff. Like, apparently, the biggest container ships out there at sea use more oil in a day then you could in a lifetime. And how NiCad batterys, the Nickel is mined in the USA, causing loads of pollution, shipped across the globe to be refined, then shipped again to be made into batterys, then shipped to car factorys, and then those cars are shipped to dealerships.

on the other hand a container ship is moving hundreds of thousands of tonnes of stuff about for that relatively small amount of fuel. 6 tonnes an hour for 150,000 tonnes of cargo ain't that bad.

Even better I've just done a quick calc based on how long I've had my car. 53,000 miles at roughly 43mpg is about 1232 gallons or 5595 litres. give or take 5.6 tonnes. So about 140years. But that is a figure based on running the engine flat out for 24hours and for a commercial ship extremely very unlikely. You could halve that by slowing the boat down and losing loads of drag.The Emma Maersk has a fairly fuel efficient engine which has a BSFC of 0.260lb/hp/hour way better than any car.
http://www.robse.dk/pages/Emma/EmmaFami.asp

maniac0796 said:

Also, what do you mean by efficiency? If I was an engine design, I'd pride myself in getting more HP and torque out of an engine with lower capacity rather than making a laborotory MPG figure.

hp/l is overrated. it exists only to cheer up owners of highly boosted cars during engine rebuilds.

Ohh, when the ships loaded, I imagine it's very effecient. But it just shows how shifting at 2.5k instead of 3k or getting another 5mpg or whatever makes feck all difference in the scale of things.

I'd personally be more interested in getting more torque, but HP would be nice.

[rev-erend]

It's been a shame that as engines have become more efficient in the last 30 years that the cars they place them in have become heavier and heavier.

Firstly comfort then crash leglislation.

[JonnyVTEC]

T0M said:

I am curious as to know whether the latest crop of modern super frugal diesel engines such as that in the VW Polo Bluemotion for example, with a combined figure of 80.9mpg are likely to be about as economical as the modern combustion engine can get?

Seems over the last 5-10 years or so the combustion engine has been really squeezed in order to increase efficiency and reduce emissions. Have we taken the biggest step in terms of effiicency out of an engine as feasibly be?

For my dissertation I have been doing some calculations based on CO2 emission output from cradle to grave between the Polo Bluemotion and Nissan Leaf. Whilst the Leaf does work out a fair bit cleaner, the only reason it is comparable at all is due to the high % fuel mix of coal and gas in UK power station. There is however greater potential for this CO2 figure to lower significantly with greater use of renewable energy and nuclear power (& commitment to EU renewable energy directive. However, the combustion engine is stuck with using oil, so the greater efficiency can only come from a higher miles / gallon ratio.

Whatcha rekon?

P.S - I'm not one of these eco warriors either, just researching for dissertation & general interest purposes. I'll take a VXR8 Bathurst Edition any day of the week

Just wondering what figures you have for the oil extraction and refining? Ive read you need 0.2 MJ to distill each 1.0MJ of fuel you get out of a refinery distillation tower.

Toyota are continuing with efficient petrol. I think the latest studies is 'concept' one with a target of 41% efficiency and they are getting 42% in testing, not sure if this involved HCCI but a later 'concept 2' is a small direct injection lean burn turbo engine.

With diesels most of the emissions equipment is likely to limit the improvments in efficiency and thats of course with the cost impact of those components.

Of course its the total system that dictates the 'efficiency' of the car. Look at the VW L1 for an example of that.

[thinfourth2]

maniac0796 said:

Then there's other stuff. Like, apparently, the biggest container ships out there at sea use more oil in a day then you could in a lifetime.

Yeah but a containeer ship can shift thousands of containers

The Emma mearsk is 14,000Teu a container lorry is 2 Teu

The real superstarts on thermal effiecency is the oil tankers as they have superlong stroke engines for economy instead of the short stroke versions on container ships. Short being 2500mm and long being 3500mm.

Couple that with a waste heat boiler and i think they are just over 50% effieceny