Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[tunnie]

Reading the Sunday Times online, with interest, there was a review on a  sported up XKR, i was amazed to see its tech specs.

Its a 4.2 supercharged V8 which does 0-60 in 4.8 and will go on to 175 mph.

Its pollution rating is 294g/km (Top band)

A 3.2 V6 in the Omega, puts out 284g/km!

The Jag has 2 extra cylinders, over 260 BHP more than the GM unit, and an extra litre in cubic capacticy.

Now the Jag engine is hardly new, its been around for decades!

Why on earth are the engines GM produce so dirty?

[cem_devecioglu]

I thinks its related to burning efficiency which GM dont care.. :-/

edit:

and another reason could be they decreased the efficiency to drop the

combustion temperature and make the engine life longer

[mantahatch]

I think it also be something to do with honesty, when searching for a replacement for the wife's Volvi T4 I looked at the emissions of many cars, and there seam to be some blatant issues with some manufacturers. bmw for instance have a 3.0 which supposedly puts out less emissions than the wife's Astra 1.8. And frankly I don't believe what some manufacturers are claiming.

Maybe GM are to honest for there own good.

Just my useless opinion.

Mike

[Elite Pete]

I think its probably to do with when the engine was developed, the GM V6 has been around for along time

[tunnie]

I think its probably to do with when the engine was developed, the GM V6 has been around for along time

Valid point, but so has the Jag V8, its the same block thats been used since 1996 that i know of, possibly even earlier?  :-/

[Vamps]

When my engine gets dirty I give it a clean.............

[Martin_1962]

It is due to the test being over 10km from a cold start.

The V6 was designed to be clean rather than pass a CO2 test

[Kevin Wood]

You can't get away from the fact that to get a given amount of power you have to burn a given amount of fuel. Nor can you get away from the fact that to burn a given amount of fuel you create a given amount of CO₂. It's just chemistry. Engine efficiency varies but not in orders of magnitude.

As Martin eludes to, the test isn't exactly representative of "normal" driving so you can tweak a car to appear very efficient whereas the reality is far from that. When the GM V6 was developed there was no need to.

Two things that stand out about the AJ-V8 are that it has an ali block, so probably warms up much quicker than the lump of pig iron that is the GM V6 block, and that it has VVT, so can probably be tuned to run more efficiently under light load, so it does better during the urban bits of the test cycle and the warmup cycle.

In reality, people drive for further than 10km and I don't doubt that if I had a 4.2 supercharged V8 I would burn more fuel , and hence create more CO₂ than I do currently.

These figures and the tests that determine them are dreamed up by bureaucrats in Brussels who have no idea what happens under the bonnet of a car. No idea about anything at all, in fact. The car industry then has to manipulate their products to get round the obstacles placed in their path.

Kevin

[cem_devecioglu]

You can't get away from the fact that to get a given amount of power you have to burn a given amount of fuel. Nor can you get away from the fact that to burn a given amount of fuel you create a given amount of CO₂. It's just chemistry. Engine efficiency varies but not in orders of magnitude.

As Martin eludes to, the test isn't exactly representative of "normal" driving so you can tweak a car to appear very efficient whereas the reality is far from that. When the GM V6 was developed there was no need to.

Two things that stand out about the AJ-V8 are that it has an ali block, so probably warms up much quicker than the lump of pig iron that is the GM V6 block, and that it has VVT, so can probably be tuned to run more efficiently under light load, so it does better during the urban bits of the test cycle and the warmup cycle.

In reality, people drive for further than 10km and I don't doubt that if I had a 4.2 supercharged V8 I would burn more fuel , and hence create more CO₂ than I do currently.

These figures and the tests that determine them are dreamed up by bureaucrats in Brussels who have no idea what happens under the bonnet of a car. No idea about anything at all, in fact. The car industry then has to manipulate their products to get round the obstacles placed in their path.

Kevin

good explanation Kevin..

the question in my mind is : does the compression ratio, engine block avg temperatures and exhaust valve timing effect  the amount of product elements :-/

[Marks DTM Calib]

Reading the Sunday Times online, with interest, there was a review on a  sported up XKR, i was amazed to see its tech specs.

Its a 4.2 supercharged V8 which does 0-60 in 4.8 and will go on to 175 mph.

Its pollution rating is 294g/km (Top band)

A 3.2 V6 in the Omega, puts out 284g/km!

The Jag has 2 extra cylinders, over 260 BHP more than the GM unit, and an extra litre in cubic capacticy.

Now the Jag engine is hardly new, its been around for decades!

Why on earth are the engines GM produce so dirty?

Which actualy shows how good a design the Gm V6 realy is.

What you have to remember its that it takes a certain amount of energy to power a car forward which will equate to a cetain amount of fuel and the CO2 emisions are calculated during the consumption tests.

The Jag is a similar weight (maybe slightly lighter), a much more complex and later design (yes it is!) yet still does not better the V6 on the output per Km.

Max power delivery has bugger all to do with the reading sadly!

Further to this, its also VERY misleading as Most cars actually do emit CO2 a fair bit higher than the tests - in fact AutoExpress measured a Focus and found it really should be 2 groups higher than its rated level!

[Marks DTM Calib]

good explanation Kevin..

the question in my mind is : does the compression ratio, engine block avg temperatures and exhaust valve timing effect  the amount of product elements :-/

Of course it does

1) If you can get the engine upto temp quicker (alloy block although this is not likely to have any significant impact as the thermal mass is not directly related to the physical mass...) then you can run at the optimum combustion temp. This is most often achieved by redudcing the amount of coolant around the block!

2) Valve timing, as per the engine basics threads, if you can keep the valve opening times at thier optimum points then you can maintain the highest possible effective compression ratio and get max consumption without affecting driveability.

3) I suspect what is hampering the jag is the supercharger, this will require a lower physical cylinder compression ratio so unless you are running full charge you will be running at a lower compression ratio.

[Marks DTM Calib]

Just to add.....the Vectra VXR (2.8V6 turbo 280bhp) has a rating of 252 g/Km.

Reality is that the CO2 value is going to be as closely related to the drag factor and vehicle weight as anything else!

[cem_devecioglu]

good explanation Kevin..

the question in my mind is : does the compression ratio, engine block avg temperatures and exhaust valve timing effect  the amount of product elements :-/

Of course it does

1) If you can get the engine upto temp quicker (alloy block although this is not likely to have any significant impact as the thermal mass is not directly related to the physical mass...) then you can run at the optimum combustion temp. This is most often achieved by redudcing the amount of coolant around the block!

2) Valve timing, as per the engine basics threads, if you can keep the valve opening times at thier optimum points then you can maintain the highest possible effective compression ratio and get max consumption without affecting driveability.

3) I suspect what is hampering the jag is the supercharger, this will require a lower physical cylinder compression ratio so unless you are running full charge you will be running at a lower compression ratio.

Thanks Professor

[Marks DTM Calib]

Take the Monaro

Its a pretty low tech (although still one of the best V8's on the planet!) engine, single cam, 2 valves per cylinder, no variable valve timing, lowish compression ratio....

386g/Km

[tunnie]

you might wanna make that avatar of yours a tad smaller