Petrol VS Diesel i'm confused!

thescouselander

Pew_Pew_Pew_Lasers! wrote: »

I suggest you do some research on this, the matter isn't nearly so black and white. For instance, petrol also produces particulate emissions that are much smaller than PM10s, and therefore more easily absorbed into the body. Petrol produces far more carbon monoxide and more hydrocarbons than equivalent diesel engines.

The nitrous oxide levels of diesels are only slightly higher than catalysed petrol engines.

I have researched this area but don't believe me check out these graphs from the DOT.

http://www.dft.gov.uk/pgr/roads/environment/research/cqvcf/emissionsfactorsforeuroiiicars?page=11

If we compare a large petrol engine with a large diesel engine at a cruising speed of around 50 kmh we can clearly see that the NOx emissions for the Diesel car are a factor of 10 higher than the petrol car (hardly a small difference).

Also particle emissions are much higher for the diesel car (again by an order of magnitude).

Thats for Euro III emissions engines anyway - things should improve somewhat with the newer IV and V standards.

Pew_Pew_Pew_Lasers!

You'll forgive me if I don't rely on graphs based purely on engine sizes, and that don't even specify the exact engine size or technologies used (turbo, common rail, etc). I am unclear as to whether they are comparing engines with catalysers or not - my diesel engine certainly has a catalyser.

Also, it is not the speed of the vehicle that matters - it is the distance covered with fuel consumed that is important.

thescouselander

Pew_Pew_Pew_Lasers! wrote: »

You'll forgive me if I don't rely on graphs based purely on engine sizes, and that don't even specify the exact engine size or technologies used (turbo, common rail, etc). I am unclear as to whether they are comparing engines with catalysers or not - my diesel engine certainly has a catalyser.

Also, it is not the speed of the vehicle that matters - it is the distance covered with fuel consumed that is important.

Yes but the graphs clearly show that the emissions for NOx and PM per km are higher for diesel over all speeds. If you download the PDF report that explains how the results were put together. The data was collected was from a number of vehicles which were catagorised into the small medium or large range. The list of cars it given at the end of the report.

There is no doubt that Diesel cars are worse for PM and NOx emissions.

chuckles1066

eslick wrote: »

We are on to our second diesel and to be honest I wont go back, 500+ miles to the tank around town and 60+ miles to the gallon on a motorways. Just dont go for a small engine one, or you will probably wish you had a petrol one especially when you pull away at round abouts or junctions.

Totally agree.

Taking a Ford Puma 1.7 petrol across to Adenkerke on a booze/ciggie daytrip (506 mile round trip) involved stopping in Calais for more petrol and then filling up again as soon as we'd got home.

Our Focus CC 2.0TD did the entire trip on a full tank (fuel warning light illuminated about 4 miles from home).

Pew_Pew_Pew_Lasers!

thescouselander wrote: »

Yes but the graphs clearly show that the emissions for NOx and PM per km are higher for diesel over all speeds. If you download the PDF report that explains how the results were put together. The data was collected was from a number of vehicles which were catagorised into the small medium or large range. The list of cars it given at the end of the report.

There is no doubt that Diesel cars are worse for PM and NOx emissions.

You should distinguish between what kinds of PMs are being emitted. You should also bear in mind that without catalytic converters the figures are very much worse for petrol engines.

Also consider that catalytic converters do not operate fully until they are hot. Most car journies are very short and barely allow enough time for the cat to get up to temperature. I very much doubt the above graphs demonstrate this.

thescouselander

Pew_Pew_Pew_Lasers! wrote: »

You should distinguish between what kinds of PMs are being emitted. You should also bear in mind that without catalytic converters the figures are very much worse for petrol engines.

What would be the point in this? All new cars sold in the UK do have catalytic converters and this has been the case since 1992. The correct functioning of this equipment is tested during the MOT. Testing a car with the catalytic converter removed would not be a representitive test.

Pew_Pew_Pew_Lasers! wrote: »

Also consider that catalytic converters do not operate fully until they are hot. Most car journies are very short and barely allow enough time for the cat to get up to temperature. I very much doubt the above graphs demonstrate this.

With regard to the warmup time for catalytic converters, on more recent cars manufacturers have taken steps to improve the situation. For example, my car has extra catalytic converters near the exhaust manifold that heat up much quicker than the main cat. Typically the catalytic converter, even on cars manufactured some years ago, can reach operating temperature in under 30 seconds.

In any case, most miles are done with the engine at full temperature.

Pew_Pew_Pew_Lasers!

....and how do you think those cats are heated up - with fresh air?

Compare fuel consumption on startup with fuel consumption at normal operating temperature. I think you'll find theres a very significant difference.

Average journey length in the UK for commuting is somewhere around 6-8 miles. A modern car even at 40mph will take around 2 miles to achieve full operating temperature. The tests you linked to did not include warm-up cycle and do not include (it would be impossible to IMO) the variety of driving conditions in the UK.

I don't disagree with everything you say but to describe what I posted as "nonsense" is something I take exception to. Modern diesel engines are very far removed from the common stereotypical view of their cleanliness.

thescouselander

Pew_Pew_Pew_Lasers! wrote: »

....and how do you think those cats are heated up - with fresh air?

Compare fuel consumption on startup with fuel consumption at normal operating temperature. I think you'll find theres a very significant difference.

Average journey length in the UK for commuting is somewhere around 6-8 miles. A modern car even at 40mph will take around 2 miles to achieve full operating temperature. The tests you linked to did not include warm-up cycle and do not include (it would be impossible to IMO) the variety of driving conditions in the UK.

I don't disagree with everything you say but to describe what I posted as "nonsense" is something I take exception to. Modern diesel engines are very far removed from the common stereotypical view of their cleanliness.

Perhaps on reflection my use of the term "nonsense" was a little strong so I appolgise for that.

I mearly wanted to to demonstrate that Diesel is not as environmentally friendly as is sometimes claimed based purely on CO2 emissions. There are pros and cons with each fuel type.

I personaly have a problem with particulate emissions from Diesel vehicles as they effect my nose in a similar way to hay fever (lots of sneazing and very runny). This caused me a lot of problems until I bought a car with a pollen filter. I have to say though, the worst offenders were busses and not cars - moden diesels seem to be very good but you sometimes see the odd smokey one especially if it is being driven fast.

Pew_Pew_Pew_Lasers!

I have it on good authority that catalytic converters on buses are often removed in-between emissions testing, to increase mpg.

Looking at some of them, I can well believe it.

pault123

drewwa wrote: »

Passat
Temperature sensor replaced (bad starting, poor economy)
Interior lights failed

Cheers,

Drew.

Drew can you elaborate on the temp sensor replacement?

I now have the Passat and very impressed with its performance and frugalness!On a full tank up to now ive done 250 miles, still with 75% fuel left and an estimation of 340 miles left in the tank :money:


BUT it does have an issue where the engine temp on the dash drops to the far left of the dial at random, I wonder if this is purely driver information or if this temp sensor actually controls other parts of the vehicle?