Natural Gas 454

[SAD90]

Hello, I am rebuilding and converting the 454 from my 78 Chevy to Cng and I am looking for suggestions. I am working with the stock engine from the truck and would like to get more power out of it. I know that natural gas is between 120 and 130 octane so it only makes sense to raise the CR, but I am not sure how high I could go, or how I should set it up. Any ideas?

[af2]

A 78 Chevy truck?

[clay]

Not sure really how the CNG systems work as far as introducing the gas. I have read that engines that have been converted to run on hydrogen wind up making less power because the gaseous hydrogen takes up so much space in the intake charge - way less room left for oxygen. I haven't heard so much about that with CNG. What I have heard about and have seen is it is rough on exhaust seats. In high school I worked for a farmer that had an irrigation system with a 351W marine engine on natural gas that ran a booster pump. The original heads lasted pretty good but after he had them redone they lasted a few days. I'm thinking maybe they did a valve job and the heads had induction hardened seats and ground through the hardness? Not sure about Fords on that. This was a steady state engine running under a pretty good load (approx. 1000 gpm at 70 p.s.i.) so yours won't be near as bad on seats but something to at least look at from the start. Clay

[SMOKEmUP]

Here's what I was able to find.

1 gge = 114,118.8 BTU's
1gge = 5.660 lbs

Therefore cng has 20,162 BTU's / per pound.

Stoichiometric is 17.4:1 air fuel ratio however most cng is not pure and therefore a stoich is about 16:1. Max power air fuel would be about 13.75:1.

I have a write up on the site comparing different fuels . From the example on that page the gasoline engine generates 53,176 BTU's of thermal energy. Running the numbers for cng I come up with 62,524 BTU's of thermal energy which is about an 18% increase in thermal energy.

In the above example the engine consumes 567.53 cfm of air. Since cng has a much higher octane rating than gasoline you can increase the compression ratio. This should make the engine more efficient and consume even more resulting in more power.

As far as compression ratio goes cng has the same octane rating as race gas. Many of the race gas engine run 14:1 - 16:1 compression ratios. I'm not sure I'd build the motor with that much compression but if you have the ability to play with the combo I'd run 12:1 compression.

Kinda makes me wish I had natural gas at my house. I bet you could have some real fun with a boosted engine on cng.

[Big Dave]

Engine doesn't consume more air and fuel. The increased compression ratio increases the combustion chamber gas temp which results in more useable work being extracted from the same charge.

I had a big block in my 1976 truck that ran dual fuel. Gas or propane depending upon which ever tank was empty. Propane and CNG are about the same (Propane is the better fuel as CNG is a mixture of Methane, and Ethane mostly). So the simpler hydrocarbons do not yield as much energy as the more complex fuels do.

In my propane truck the propane gas carburetor sat inside of the air filter and dumped a mixture of vaporized (cold and dense) propane and air mixture on top of my Holley carb. There was a valve that was heated by the engine coolant to vaporize the liquid fuel that entered the valve at reduced pressure from a primary valve on the propane tank in the truck bed (similar to the pressure regulator on your air compressor that limits your line pressure to less than the tank pressure).

Aside from the fairly large valve and bracket to bolt it to the engine (about the size of an alternator) you wouldn't know I was adding gas to the mix. I have also seen mechanical fuel injectors (Hillborne) that burnt straight liquid propane from the tank by using a machined pill to meter the fuel to air ratio combined with the usual ramp valve attached to the throttle plates. Like old fashioned mechanical injectors it was cranky and hard to start. Though I imagine an EFI system would be much easier to work out on the street.

Big Dave

[SMOKEmUP]

[Big Dave]

Volumetric efficiency isn't improved by higher static compression (bigger domes). What does affect it are your choice of stroke, cam timing and your exhaust system's size and back pressure, then any induction improvements will further affect your volumetric efficiency: but a static compression increase will not. Increasing the compression increases your horse power by about 4% for each full number of increased compression because you are increasing the separation of the top of the Otto cycle's curve from it's base (the heat sink temp). That added area under the curve is your increased power. The amount of gas and air burned remains constant you can just extract more power from the engine because of the increased heat.

From thermodynamics, the Otto cycle uses Nitrogen gas as the fluid that is cyclical heated to expand and cooled to contract with each cycle.Increasing compression adds heat that adds power (free power, as there is no mechanical losses as there are with many power adders just more pollution in the form of nitrates).

Big Dave

[SMOKEmUP]

I disagree. If you have an engine and only vary the compression ratio the amount of air consumed will vary.

If you compare two engines one with 8:1 CR vs a 10:1 CR, everything being the same the 10:1 engine will make more power. If the 10:1 engine is making more power it's burning more fuel and to burn more fuel it needs more air. Therefore VE has increased since the base engine volume has not changed.

[SAD90]

Yes it is a 78 crew cab Chevy. I have heard about the valves before. I guess there is really no lubrication since its a gaseous fuel. I don't know if there is much I could do about that other than getting hardened valve seats. As for the compression ratio, I was thinking 13:1. I want to make as much power as I can, but this is my daily driver. Do you think this is too high? I know if you don't go high enough with the CR, cng tends to run hot because of the high octane level. I might want to add boost later on so I have to consider that also. Anyway, How would I achieve that high of compression? I'm sure I'll have to go with domed pistons and smaller chamber heads, but I don't really know any specific numbers. My crank is junk so maybe I'll factor in a stroker crank as well.
Thanks for the replies.

[af2]

[10sec.et]

[SMOKEmUP]

[Big Dave]

I have had five courses in thermodynamics, but you needn't spend two and half years of your life in a class room when this article explains it fairly directly. Note that the quantity of fuel and air are a constant and the only thing that changes with increased static compression is the amount of power that is extracted from the fuel (which is generally more completely burnt which is good for emissions, just that it occurs at higher temperatures which involves nitrated forming which is bad for emissions).

http://www.popularhotrodding.com/enginemasters/articles/hardcore/0606em_understanding_compression_ratio/viewall.html

Another article that isn't as easy to follow, but it is a good reference if you already understand what he is trying to say.

http://www.popularhotrodding.com/tech/0311_phr_compression_ratio_tech/

And if you are curious what an engineering text looks like here is a chapter out of the book talking about the second law.

http://www.uccs.edu/~rtirado/Ch19%20ISM.pdf

Big Dave

[10sec.et]

this is definitely not an Olds board. if it was, we would be calling each other nasty names by the fifth post.

now i have to read Daves links to form an intellegent response. thanks for the homework Dave .......

[Big Dave]

I wouldn't read the engineering text as it will require calculus to follow the differential equations, not to mention it will put you to sleep after about five pages with your eyes glazing over after only two pages.

Big Dave