diesel, gasoline natural gas fuels differences

[grakson]

Hi,

I know something about thermodynamic, fuel physical characteristics, so please try that answers/discusion will be of any use.

I tried to discuss about the topic on the other automotive forums, but people seem to know something and mostly I got cross-explanation of my former statements.

I would like to discuss (based on the thermodynamic laws), why some fuels are suitable for a particular engine design and other not or might be with some changes and modifications.

Nowadays manufacturers offer also CNG vehicles, especialy in a transit chategory.

So I will give some of my thinking, whic is really bunch of questions.

IF we put diesel fuel into an otto design engine, it probably won't run, due to the fact that there won't be enough air in the cylinder and also the problem will be the fuel vaporization as diesel fuel is a mixture of long carbon-hydrogen molecules and has low viscosity. But I thing that also the big problem will be autodetonation rather that spark, causing engine knock. Diesel autoignition point is lower that that of the gasoline.

If we put gasoline into diesel cylce engine, following will probabl occur. Due to the fact that gasoline is more viscosity, and its speed and temperature of burning is higher, it will cause explosive effect. The fuel will reacted with the oxygen too fast causing to high peak temperature and pressure, probably damage the engine.

What about monogas CNG engines. Its main component is methane which has high autoignition point (round 630°C), thus allowing manufacturers to build engines with higher compression ratio, up to 16 or 17. (I make this estimation based on the thermodynamic equation T2[K]=T1*(V1/V2)^k-1 k(air)~1,4). But as I can read NGV vehicles (Honda civic NGV=12,5), have lower compression, why?

Could we build higly efficient CNG engine with very high compression ratios? Will the problem be peak burning temperature?

Could we use diesel cylce engine to run on gasoline or CNG, where we will inject fuel stratified - meaning more time subordinate partly injections of fuel?

 

[Lou Scannon]

I suggest you get yourself a book on reciprocating engine theory and design. Use the google search function above to search for discussion of the books by Heywood, and by Taylor.

 

[grakson]

"But I thing that also the big problem will be autodetonation rather that spark, causing engine knock. Diesel autoignition point is lower that that of the gasoline."

sorry for my mistake. I miss-checked the fuels characteristics table and mix gasoline and diesel numbers.
According to the table gasoline has lower autoignition point - (495F), when diesel #2 is (~600F)

 

[dcasto]

I burn natural gas at work, google cat 3616. Methane is just like gasoline. It has an octane rating of over 100 so a high compression ratio of 12 to 1 is OK. But we use low compression ratios and turbochargers to booster efficency. The down side is that unlike gasoline, we do not get the heat of vaporization to cool the cylinder before compression is complete.

Ignition temperature isn't the sole factor in the engine, the kenetics have some effect too. But, more importantly, you forgot about the air in the cylinder. Air being compresed 16 to 1 will approach 900 F.

 

[Bribyk]

You have no air charge cooling, due to evaporation, with natural gas; which gives higher in-cylinder and exhaust temperatures. Industrial gas engines have compression ratios in the range of 8.5-10.5 but are typically turbocharged and intercooled. Light-vehicle engines aren't designed to last as long so you could bump the compression up a little before you starting burning up exhaust valves, heads and pistons. Natural gas will cause knock as well (its autoignition temp is around 1200degF) due to higher cylinder temps.

Diesel won't burn well when spark-ignited and gasoline would knock horribly when compression-ignited because it burns much more rapidly.

 

[grakson]

latent heat of evaporization of gasoline has wery small inpact on compression temperature due to small amounts of gasoline versus air.

At stoichometric point there is only 6,7% of gasoline and gasoline's latent heat value is much lower than water's, so due to latent evaporization effect the overall compression temp may decrease up to 5%. So if we use water+gasoline it may have some small effect but at sole gasoline it is not a big deal.

On the other hand CNG's storage pressure is 200-220 bar, and when extracted to the atmospheric or boost pressure it gets quiet cool.
So if we calculate what does cool the engine's compressed air more it will be CNG.

And CNG isn't like gasoline, its octane rating is much higher, and according to my calculations temp under 16: to 1 compression won't exceed NG autoignition point.

Of course other is high burning temp, but I don't think that due to high compression brning temperature will be so much higher.
Burnign temperature is dependant mostly on the amount of oxygen in the air and the amount of air pressure and amount of fuel vapour in the cylinder.

 

[Bribyk]

According to my calculations and NIST data, isenthalpically throttling methane from 200 bar g to 1 bar drops the temp from 68 degF to -89.5 degF. When you mix this with air at 68 degF at a stoichiometric AF (9.53:1) your final charge temp is still 51.4 degF, neglecting any reaction.

If you evaporate gasoline in a stoich mixture (14.7:1), the latent heat alone drops the air temperature by 42.8 degF (from 68 to 25.2 degF). I didn't bother including the air heat lost to sensibly heating the fuel vapor. Ever seen a carb ice up?

Then when they are compressed in the cylinder according to T2/T1 = CR^(1.4-1) at a compression ratio of, say, CR=12. Your 26.2 degF charge temperature difference between the two fuels results in a compressed mixture temperature difference of 72 degF before the fuel is even ignited. Not to mention that you've got more air in the cylinder to burn because it's denser...

 

[dcasto]

But you can't count on 200 bar all the time. Instead they warm the CNG up so it above -40.

You won't get 16 to 1 before you get detination, it's -21 out and our engines are on control. One little change in the mole percent of ethane (ie lower octane) and the units will alarm on detonation.

The latest use of methane is to fumagate diesel with LNG in truck engines