mart360

Ok as far as i am aware, diesel comes fairly early on in the cracking process??

petrol being one of the last to arrive..

if thats the case, why does diesel cost so much more than petrol??


and why are diesel engines so economic, is the same amount of fuel injected as per a petrol engine?? or is it less, if so why isnt the diesel technology applied to petrol, sureley if you compress petrol vapour that much, it should ignite under its own heat??

Mart

fast bloke

tax and det mostly

fatherpierre

Tax as per last post.

2nd, oil burners run at higher pressure so are more economical. It's more comlex than that, but that's the simple answer.

mark1234

Diesel has a somewhat higher calorific value than petrol (more stored energy)

Can't comment on cracking process, but petrol is a shorter hydrocarbon chain (lighter) than diesel. It comes off first in distillation, whether that's relevant I'm not sure!

Yes, petrol does ignite under it's own heat.. a lot earlier than diesel. It's called detonation or pre-ignition, and it destroys scooby engines It's also why there are limitations on compression ratios for petrol engines. Seriously, the source of ignition is irrelevant.. it still burns the same.

Same reason diesels don't rev; they can't advance the ignition, and the fuel burns too slow..

brickboy

Apparently the refiners say that "producing more diesel takes time and adjustment to processes". Yeah, right

They're simply making hay while UK drivers switch to diesel.

logiclee

Diesel is also a lot more thermally efficient due to it's slow burning. More of the fuel burnt goes on driving the car and less on wasted heat. Increase thermal efficiency and you use less fuel per bhp.

Only downside is modern diesels take an age to warm the heater up in winter unless they are fitted with addition pre-heaters or diesel burning heat exchangers.

The VAG 90/110 engine still holds the record for the most thermally efficient engine fitted to a production road car. Probably why all the taxi drivers are now buying Octavia's, 50-60mpg and 2 years/30000 mile servicing.

Cheers
Lee

CrisPDuk

Diesel used to be cheaper, it was a government incentive to get more people buying diesel engined cares. Change of government occurs, G Brown sees an excellent way of easily screwing the nasty car users and ramps up the duty

Thanks to massive advances in engine management technology, manufacturers are now starting to produce direct injection petrol engines. The problem with petrol is that it is much more volatile and less chemically consistent than diesel, making it much harder to develop a reliable and efficient compression ignition engine. It is only now with modern sensing and processing systems that it becomes a viable prospect.

I've noticed that too. Two years ago all the taxis were Toyota Avensis', now you almost always get ferried home in a half price Audi

ALi-B

iTrader: (1)

Firstly...go to spain!...diesel is MUCH cheaper than petrol over there - that should answer the first question


The second part is a bit tricky and there is no specific answer. It's only in the past 5 years that diesel technology has moved on properly in terms of making use of current technology. Previous diesel engines haven't really changed that much since the 70's. And most of the recent advances are ideas taken straight off petrol engine:

That being:

Multiple inlet and exhaust valves (four or five valves per cylinder)
Crossflow cylinder head designs
Electronically controlled fuel injectors
alloy heads and blocks and other internals as opposed to iron.

Are a few that spring to mind all were on petrol enginesfor deacdes whilst diesels never benfitted from it until recently.


Moving on to petrol engines, one of the biggest confines compared to diesel is maximum compression ratio that can be used without inducing detonation, that combined with fuel mixture means that a petrol engine HAS to have a reduced compresson ratio and cannot run too lean, otherwise it'll sound like bag of nails and wouldn't last very long (overheating, melted pistons kackered big ends etc). This has the biggest impact on both power and economy. Another limiting factor is the fuel itself being its octane rating, the lower it is, the less compression (and lower boost - for turbos ) and richer you have to run the engine.

Diesels can run infintely lean with little consequence (except power). So can very high compression ratios and boost pressure yet still run very lean.

However, petrol engine designs and control systems have evolved to allow higher compression ratios and leaner mixtures. A good example is the May "fireball" combustion chamber on Jaguar's V12 engine, combined with the ignition and fueling control, is somthing very much ahead of its time. Which even modern engine struggle to better. Pent roof combustion chambers and electronic ignition/injection with knock control is other means to allow higher compression and leaner running - but to a point.

Some diesel technology is starting to appear in petrol engines though, that being direct injection. So far, not many manufacturers have adopted it (Mitsubishi GDi is the only one I know of in current production ). Potentionally it does allow for much more detonation control while using a lean fuel mixture and higher compression ratios.

TonyFlow

TBH, I think Diesel technology has been moving along fairly rapidly, maybe not in passenger cars, but HGV's have come on leaps and bounds since the 70's. In fact, pretty much all improvements to passenger DERV's have come from what HGV's have been running years before!

logiclee

I think Ford have a 1.8 direct injection petrol engine available in the Mondeo.

Most diesels actually run without a throttle so the full amount of air is entering the engine all the time. Even Turbo Diesels will run with full boost without full fuel.

It doesn't matter you have all the air in the cylinder and just add the amount of diesel you would like to burn in that air.
Diesels will also run with excess air to cut down on particulates and may even recycle unburnt products of combustion back through the cylinders.
It's why diesels will get upto 30% power increase from a little cheap box that ups the fueling. The extra air is aleady there, you just add more fuel for more power. If you go to far you will get smoke but most TDi engines will go up 20-25% without visible smoke.

Cheers
Lee

mart360

Thanks for the replys guys..

we looked at a mitsi gdi before we bought the seat, but wernt to sure on just how good it was.

re the power increase, i had heard on the vag 90/110 engines, a free boost can be realised just by changing the injectors to the next size up, this just pumps more fuel into the chamber this would be reinforced by what you have said here..

it seems surprising that given all the research done, petrol still suffers from poor returns in the oomph dept when being used... yet diesel gives a better return..

i know what you mean about taking an age to warm up, is that due to the engine being iron block / head etc..

another question? what is the purpose of a cat on a diesel, i,m not sure but i believe my engine has one, again this asks the question, is it needed and does it effect the emmisions/ does it work in the same way as as on a petrol varient..


Interesting remark about having no ignition advance or retard, how does the engine overcome this??


Mart

CrisPDuk

It's more to do with Diesel burning at a lower temperature than petrol. When that is combined with the engine more often than not running lean in a normal urban cycle, you get the following: little or no fuel entering the combustion chamber means there is next to combustion, no combustion means no generated heat, no engine heat means no cabin heat

logiclee

No need to go that far, a £200 tuning box will keep the standard injectors open longer and increase fueling upto the air available. Stick in a free flow air filter and a 35bhp increase is possible without much visible smoke.

Down to the thermal efficiency of diesel compared to petrol more bhp and less heat from the amount of fuel used. All down to the burning charachteristics of the fuels.

No mostly due to thermal efficiency again, not much wasted heat from the fuel and diesel engines don't need to run overly rich in the warm up period.

Injection timing can be altered but diesel burns rather than ignite the fuel so it can be progressively injected rather than all put in in one shot, this also cuts down on diesel noise.

Cheers
Lee

logiclee

Anyone seen the specs on the new Audi A8 4.2TDi V8.

321bhp 479lbft, weighs 2 tons and has an auto box but will hit 62 in 5.9, electronically limited to 155mph and will still do 39.8mpg on the extra urban cycle.

Merc and BMW also have triple turbo models in the pipeline.

Cheers
Lee

mark1234

Are you saying that DI diesel engines are effectively two stroke? they're injecting as they're firing?

I accept I may be proved wrong, but I was rather of the impression that they're 4 stroke, and the fueling is present in the compression cycle, NOT injected during it..

FWIW even petrol 'burns', rather than explodes - when it doesn't burn a clean flame front, and ignites randomly is when you have det, and it goes a bit wrong. The difference in propogation of that flame front may be significant, but in either case, it's bloody fast!

One way timing could (potentially) be altered is via a variable compression engine.. but there's still going to be a practical limit on how fast it can rev, whilst still alowing time for the fuel to burn.

As for other DI petrol engines... Audi FSI I believe?

mart360

given as you say the turbo is on full boost all of the time, how do you confirm this??

i feel the turbo pulling from 2k, but unlike the last turbo,s i have driven the acceleration is not as brisk so to speak, but it more of a linear acceleration..

my saab was explosive acceleration, as are the 2.0 cmaxed tdi,s i have driven

the other turbo tdi (90bhp) i drove was very eager, mine feels more sedate but has more longevity through the rev range..
mines also a 90bhp

M