Difference Between Diesel Car & CNG Car

Are there CNG fueling stations everywhere you would want to go in this car?

thanks for reply....yes it is there....

What sort of annual mileage - 3000? 10000? 50000?

We have a member who advocates LPG, based on the lower cost per mile of it as opposed to diesel.

• One disadvantage is the lower flashpoint of CNG.
• Another would be the energy content per unit weight.

Until some more parameters about the vehicle and its intended use can be revealed, the best that can be done is a guessing game.

Coincidentally, I have just finished reading EU Directive 2009/33/EC (my cup runneth over!). Table 1 of the Annex gives a list of energy content of various fuels. Here are a few to be going along:

Diesel: 36 MJ/litre

Petrol: 32 MJ/litre

Natural Gas: 33-38 MJ/litre

LPG: 24 MJ/litre

For those who want more, the Directive can be found at the European website

Which beautifully illustrates why gas/petrol car engines/conversions have a far higher fuel consumption than when driven on gas alone.....and have to lug around heavy gas cylinders as well.....

That is further reason to my mind never to buy a gas conversion for a petrol engined car.

The benefits for diesel engines burning gas are also mentioned here:-

http://www.ehow.com/list_5885556_propane-injection-benefits.html

Maybe we should build a list of the good and bad points, so I will start here:-

Bad Points

1) Increases car weight substantially.

2) Can possibly produce a dangerous explosion if a gas connection gets damaged or knocked off in an accident.

3) Gas price is substantially higher here (like petrol) than last year, it is about 35 % more.

4) It would appear that guarantees from both car manufacturer and gas converter firms do not protect the owner in the event of problems. Each blaming the other....

5) Gas burns hotter and on converted cars it is often not allowed in many cases to drive with a trailer or even legally heavily load for example. See the small print. This has been also used to disallow a guarantee, in spite of customer saying the opposite...or not even having a tow hitch....

6) Reduced storage space in the VEHICLE due to the storage tanks.

7) Severely curtailed driving range when fueled only bay gas, though of course the petrol tank is still available if required.

8) Some countries still do not have full gas coverage.

9) Possible long term engine damage/ short life due to hotter burning fuel. I could not find any figures to show this clearly. Mostly I found only conversion companies declaring increased engine life...But I did find a company that appears to sell products to reduce the negative effects of hot burning gas on the engine valves, a really "hot" component:-

http://www.autoprofishop.de/product_info.php/language/en/info/p501_set-ventilschutz-valve-lube---lubrication-system-tropfadapter.html

Good Points

1) Some savings on fuel costs, so for those able to do high mileage per year on gas, they will claw back the installation costs provided that the conversion has a) a good guarantee and after sales service and b) Reliability is good.

From a driveability point of view, diesel powered vehicles can usually travel much farther before refuelling than an equivalent CNG powered vehicle. Most modern diesels also have more power (both torque & bhp) than an equivalent CNG, although this may not be an issue especially if the CNG being compared is larger displacement.

It really depends how the 2 compare in price, fuel costs, your driving style & availibility of diesel in your area as well.I wouldn't consider cold weather to be an issue for modern diesels.

"One of the biggest complaints about NGVs is that they aren't as roomy as gasoline cars. This is because NGVs have to give up precious cargo and trunk space to accommodate the fuel storage cylinders. Not only that, these cylinders can be expensive to design and build -- a contributing factor to the higher overall costs of a natural-gas vehicle compared to a gasoline-powered car. Another drawback is the limited driving range of NGVs, which is typically about half that of a gasoline-powered vehicle. For example, Honda's natural gas Civic, the Civic GX, can go up to 220 miles without refueling. A typical gasoline-powered Civic can go approximately 350 miles without refueling. If a dedicated NGV ran out of fuel on the road, it would have to be towed to the owner's home or to a local natural gas refueling station, which might be harder to find than a "regular" gas station.

Finally, it should be noted that natural gas, like gasoline, is a fossil fuel and cannot be considered a renewable resource. "

http://auto.howstuffworks.com/fuel-efficiency/alternative-fuels/ngv4.htm

Isn't a CNG vehicel always powered by both fuels? If cars get retrofitted with CNG they still can run on petrol and I am not sure even on Diesel. If efficiency is not as good with a CNG capable engineene but my understanding is that the range is actually higher than without CNG. Its only maybe not so efficient enough anymore to drive on the secondary fuel.

A diesel engine is markedly different from a petrol engine. While it is true that a Petrol engine can run CNG or LPG and a diesel can run on LPG or CNG, you can't run a petrol engine on diesel and vice versa. I'm not an expert on using LPG or CNG in diesels, I know CNG is now used a lot on city busses, so I'm guessing it is fairly feasible. I understand there are some risks with using LPG (and possibly CNG) in a diesel engine under a heavy load or trying for a lot of horsepower. Someone more knowledgeable than I will hopefully fill in the gaps there.

At my previous place of employment we had a diesel/LPG Transit. It was fairly low powered (and old), so there wasn't much difference in performance between fuels, although I suspect both were less efficient than they might have been.

It got, afaicr, around 150 miles on the LPG tank and 250-300 miles on the deisel tank.

The major issue I know of in using LPG in a diesel engine is the lack of lubrication provided by the LPG; diesel is a lubricant and the engines are designed around using this property. As I recall, the advice is to make sure you run it on diesel regularly to ensure a level of lubrication (you get some residual lubrication that lasts when running with LPG). You don't get this issue in a petrol engine since petrol is a solvent and so the lubrication arrangements in a petrol engine are different to those in a diesel engine to take this into account.

The gas does not go through any diesel component other than the engine itself on gas/diesel engines, so the lubrication problem does not exist. It does burn hotter, so a new radiator may be needed when converting.

This problem can exist in diesels that use special additives in the tank for winter driving, many people used petrol which does not lubricate...not recommended.

Diesel is still used when using a gas conversion to ignite the gas, otherwise a spark plug would be needed to ignite it. Basically, the amount of diesel needed at each firing stroke is about the same as needed for ticking over....a small amount only.

Gas petrol conversions have had a mixed life/reputation here. Gas is getting very expensive, gas powered engines, especially in a petrol conversions run hotter than when on petrol, towing using gas is often not allowed by the conversion companies in their small print....gas usage is much higher than petrol usage in the same car/engine, which can offset a large part of the price differences between the two fuels. 10-15% higher consumption is quite normal.

Engines specifically designed and built to run on gas are not usually a problem. Ford for example have such engines.

Gas is even more dangerous than petrol in the event of damage to the high pressure cylinders, though they are very well constructed in most cases.

Gas cars are often not allowed in many underground and some over ground car parks.

Diesel only burns when raised to around 50°C or higher, and never (though one should never say never!) explodes in accident situations like both gas and petrol can. Therefore its a much safer fuel.

Diesel engines tend to be very long lived and can achieve higher resale values (here in Europe) than petrol engined cars of the same type, provided that the engine oil and the filter are of good quality. Also Diesels are simpler and generally have fewer electronic components and therefore failures when used and maintained correctly.

Gas powered cars here tend to dramatically lose value when resold here, its often a good idea to get the conversion removed before sale.....it actually improves the value of the car...almost unbelievable.....

I am not convinced that a gas car is cheaper to run/use than a diesel is, its certainly liable for more problems and the possibility of falling between the engine manufacturer's guarantee and the conversion company's guarantee and ending up with no guarantee and a broken engine.....

If you must have gas, buy new from a company that has a purpose built engine for it like Ford for example.....

I would love to try out a diesel/gas conversion myself......

Thanks Andy - good points there. I'm a diesel fan myself. I haven't been able to see an economic advantage in converting to gas - although as you say, purpose designed engines would be superior.

On the lube side, I was told (and never bothered checking! Tut tut) that the gas tends to remove the oil residues in the cyclinders, resulting in more wear if run "purely" on gas for long periods. Maybe it's more the higher temperatures that tend to burn more of the oil off?

The normal CNG/LPG Diesel conversion does not use a spark plug for ignition, it still injects a small amount of diesel into each cylinder at TDC, the same as when the engine is running on diesel alone. This ignites the gas that enters via the air intake (no extra injector required).

But I do not believe that any diesel oil contributes to any lubrication of the engine anyway, so should not be a problem. Diesel does lubricate the injectors and the injector pumps though....

The rings on the piston are responsible for oiling the cylinder walls using lubrication oil, but scraping most of it off again before combustion. Here is a reasonable explanation:-

http://courses.washington.edu/engr100/Section_Wei/engine/UofWindsorManual/Piston%20and%20Piston%20Rings.htm

On both diesel and petrol engines, with and without gas, there should be almost no oil on the cylinder walls in the area when combustion takes place. Also, the piston is either at TDC or almost at TDC, so very little of the cylinder wall/oil, if any, is exposed to the flame of combustion.

If there is oil left in quantity in the combustion area (and no diesel filter or cat), you will see a strong blue tinge to the exhaust. It called "burning lube oil". Too much fuel, either diesel or petrol, will leave a black tinge to the exhaust. I would guess that gas might be the same, although the flame front is actually much faster with gas......

The oil that is normally left on the top of the cylinder wall and is exposed to the flame of combustion, is an extremely thin coat, is basically the oil that an engine uses in normal driving. My diesel car for example uses about 0.5 liter in 20,000 KMs. This is simply not worth thinking about and would be considered minimal and normal.

In my limited experience, diesel engines in a good condition, almost always use a small tiny amount of oil in normal use, some petrol engines use so little oil that it is difficult to actually see any usage at all between oil changes. I am of the belief, since I was a teenager, that the initial 5-10,000 KMs of driving style of a new engine/car often set the oil usage.

VW in the late 90's forbid in the owner's manual any towing or high speed driving in the first 5,000 KMs......I have not seen it in any recent handbooks (but that is not saying that its NOT there). They seem to agree with me.....at least then.....

The old Guy that taught me (and bored and rebuilt my engines at that time) instructed me to set an engine up, with as little running as possible, fill the tank and go for a 250 mile or more run, not switching the engine off, no high revving etc., taking food and water with and do it all at 30-40 MPH. The engines I treated so demonstrated a very low oil usage in normal use later and higher power (unchecked on a Dyno, but they were demonstrably faster than otherwise identical cars). So I still do it.....even on previous company cars.....

Converting a diesel engine to use LPG/CNG is scarcely worth the effort.

http://www.go-lpg.co.uk/diesel.html

For the type of conversion he is talking about, he is right. But what a waste of time and energy, turning a diesel into a gas or petrol engine! Laughable.....

Sadly he does not comment fully on the "normal" gas conversion for diesels, often buses, which not only save money, but also clean up some diesel's exhaust dramatically.

I looked around for more detail and these links show exactly what needs to be done:-

http://www.ozonlpg.com/diesellpg.html

http://www.dieselgasaustralia.com.au/default.aspx?ID=Technical

http://www.lpgconversiondirect.com.au/diesel-conversion/

http://outbacktravelaustralia.com.au/doc/LPG_plus_Diesel.pdf

Although diesel exhaust filters have really also done this on some cars. I recently drove an Audi Diesel with 160,000 KMs on the clock, the original exhaust and it was clean blank metal inside. No trace of any exhaust products....

Hey Andy,

Granted converting a gas or diesel engine over to natural gas is unusual but.....

one of the more interesting challenges I had to deal with at JD was converting 12 type 6101 6cylinder diesel engines to run on dirty natural gas. They were bound for a sugar cane plantation in South America that also had a producing oil field on it so they wanted to use the "waste" natural gas to run these engines that were going to run irrigation pumps periodically. Took me and the folks at Aerodyne over a month to just get the fuel pump right alone. The injectors were a whole nother story. LOL Ended up having to install a reserve oil tank and "oil scrubbers" on each install because Dirty natural gas was a real understatement. In a brief aside, I will never again travel to a "undeveloped" country. Worst three weeks of my life.

Interesting post....many thanks.

Here the man gets a company paid vacation to an exotic location, and all he does is complain.

After reading all of the good posts, if you still have not decided which way to go, try this. Long range miles per dollar, or whatever monetary equivalent you use.

diesel engines ignite the fuel from the heat produced from a high compression ratio. a spark is not needed.

WOW, are you a well informed Bunny, this blog has been dramatically improved with your post and outstanding knowledge of the subject in hand....

.....and who here did not know that already?

Anyone? Please put your hands up!

Why are there no hands?

Thank goodness you had the extreme confidence to speak up and tell the world like it is. I wanted to say that, but I was too bashful. Your courage is admirable. I wish I had your guts, Mr. Anonymous Poster. Your family must be so proud of you!

thanks for the sarcasm. i'll have the wife hand cuff me to the bed posts and whip me for being a stupid boy.

Any pictures please do not post!

Wow! Does that sorta explain the "durtie" part?

Some will be jealous here.....not me though....but each to his own fun.....no critique at all.

They used to call it the "English disease" or similar I believe many years ago.....absolutely no idea why....perhaps someone can enlighten us further....Wiki was not helpful.....

Now you know what I occasionally read as a teenager....OOOOPPPPSSSSS!!!!!!

Have a GREAT weekend....!OW!!OW!!OW!!OW!!OW!!OW!