LarryFR
CS,CO,USA
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Boy, maybe I shouldn't jump in here, but I will.
Bert talked about the variable air/fuel ratios of diesel engine vs the constant ratios of gasoline engines.
While true in a general sense, they both have variable fuel ratios, the diesel is just MORE variable.
For optimum combustion any internal combustion engine must be run at an ideal air/fuel ratio. Ratios that are leaner produce less power and more emissions, ratios that are richer produce more power (within limits) and more emissions. The ideal mixture for gasoline and diesel engines is almost the same. Gasoline being 14.7:1 and diesel 14.5:1. See Website. Near the bottom of the page is a chart that shows the relative ideal and max power ratios for various fuels.
From the chart it would seem that gasoline would be more efficient than diesel because the air/fuel ratio is more favorable for gas. The diesel uses less fuel to deliver the same power because there is about 20% more energy per gallon in diesel than in gasoline. So if they are consuming the same amount of air, the diesel makes more power because if the superior energy density of the fuel. Conversely, if they are making the same power, the gasoline engine burns more fuel.
So is a diesel air/fuel ratio 100:1? Sometimes, but at full power it is more like 11:1, just like the gas engine.
Larry
'88 Bounder on a P30, Gear Vendors, Thorley and other goodies.
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BertP
Edmonton, Alberta, Canada
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Quote:Boy, maybe I shouldn't jump in here, but I will.
Not to worry, Larry ![[emoticon]](http://www.coastresorts.com/sharedcontent/cfb/images/biggrin.gif "biggrin")
Quote:Bert talked about the variable air/fuel ratios of diesel engine vs the constant ratios of gasoline engines.
Minor correction: I didn't say that a gasser has a constant mixture, just that it does not tolerate much deviation. Does that mean that it is variable? Yes. If you look at general aviation aircraft with piston engines, standard procedure is to lean the engine once cruise has been established. That is done partly to compensate for the lower density at altitude and partly to reduce fuel consumption. Care must be taken, though, because if you lean too much, EGT and cylinder head temp both rise and, if you leave it long enough, you will burn holes in the pistons.
But, you can only lean so far. For example, to shut the engine off in a piston aircraft, you do not turn the ignition off. Instead, you lean the engine to the point that it will no longer run. Then you turn the ignition off.
Quote:While true in a general sense, they both have variable fuel ratios, the diesel is just MORE variable.
True and that is the point I was trying to make.
Quote:For optimum combustion any internal combustion engine must be run at an ideal air/fuel ratio. Ratios that are leaner produce less power and more emissions, ratios that are richer produce more power (within limits) and more emissions. The ideal mixture for gasoline and diesel engines is almost the same. Gasoline being 14.7:1 and diesel 14.5:1. See Website. Near the bottom of the page is a chart that shows the relative ideal and max power ratios for various fuels.
Thanks for the link. That is the type of site I was looking for.
Quote:From the chart it would seem that gasoline would be more efficient than diesel because the air/fuel ratio is more favorable for gas. The diesel uses less fuel to deliver the same power because there is about 20% more energy per gallon in diesel than in gasoline. So if they are consuming the same amount of air, the diesel makes more power because if the superior energy density of the fuel. Conversely, if they are making the same power, the gasoline engine burns more fuel.
So is a diesel air/fuel ratio 100:1? Sometimes, but at full power it is more like 11:1, just like the gas engine.
Larry
Diesels and gassers work on exactly the same principle, so your findings are not surprising. I have to admit, though, that I have wondered what would happen if you took a gasser and adapted a fuel delivery system from a diesel for it. One of the biggest problems a gasser faces is detonation. Since the fuel system on a diesel engine delivers the fuel to the cylinder when it is needed, it seems that detonation would be avoided. Anyone know if that has been tried?
Bert
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Aquaduct
Winchester, VA USA
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Quote:
Diesels and gassers work on exactly the same principle, so your findings are not surprising. I have to admit, though, that I have wondered what would happen if you took a gasser and adapted a fuel delivery system from a diesel for it. One of the biggest problems a gasser faces is detonation. Since the fuel system on a diesel engine delivers the fuel to the cylinder when it is needed, it seems that detonation would be avoided. Anyone know if that has been tried?
Bert
No, they're different combustion regimes. Gassers have a pre-mix in the intake manifold and then they burn with a homogenous flame front. Diesels have stratified, non-homogenous combustion. Little burning droplets, if you will. It means that you're fighting very different problems.
As far as air/fuel, these days you're limited by emissions requirements. The extra heat of lean combustion forms NOx. Rich mixtures leave HC and CO in gassers and soot (particulate matter) in diesels.
Gassers have the miracle of Three Way Catalysts to control emissions, but to do that, you've got to stay very close to stoich. Basically, the TWC switches very fast between oxidation (HC and CO) and reduction (NOx) regimes to stay very clean. You could lean out a gasser to get better fuel economy, but you won't have a prayer of a chance of emission certification. Since you get the proper mix prior to ignition, it's easy to run at stoich.
Diesels have a different story. Since you're injecting fuel droplets, you've got to run lean to make sure you've got enough oxygen for combustion. You've also got to have as small a droplet as possible, thus huge injection pressures. Without enough oxygen, the droplets only burn to little "charcoal briquets" which is known as particulate matter. So in reality, you can only run to about 18:1 before you belch too much black smoke. I think that momentarily diesels can even exceed 100:1, but most of the time these days, you're about 18-25:1. That makes it real tough to get catalysis to work. If you're lean, you're producing NOx. To get rid of NOx, you need to reduce it or take the oxygen away. Tough to reduce things in an oxygen rich environment.
It's kind of ironic really. As technology progresses, gassers try to figure out ways to run lean for better fuel economy. Diesels try to figure out how to get to stoich for emissions reduction.
It makes for a fun work day!
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BertP
Edmonton, Alberta, Canada
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Quote:No, they're different combustion regimes. Gassers have a pre-mix in the intake manifold and then they burn with a homogenous flame front. Diesels have stratified, non-homogenous combustion. Little burning droplets, if you will. It means that you're fighting very different problems.
Thanks, Pete. I figured it was too simple and obvious a solution to have been overlooked but I had to ask ![[emoticon]](http://www.coastresorts.com/sharedcontent/cfb/images/smile.gif "smile")
I am curious, though. Why can't you inject the fuel directly into the cumbusion chamber in a gasser the way you do in a diesel? Is there a problem with the way it would burn? I seem to remember reading that detonation is basicly a really messed up flame front instead of a smooth one. Would direct injection of a gasser cause that kind of a problem?
Bert
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tgatch
Meridian, ID
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Joined: 08/13/2003
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Quote:I am curious, though. Why can't you inject the fuel directly into the cumbusion chamber in a gasser the way you do in a diesel?
They do directly inject into the cylinder. They call it port injection. My 4.0L Jeep had six injectors, one for each cylinder. The injectors just don't inject at the top of the combustion cycle like a diesel does.
Tom
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BertP
Edmonton, Alberta, Canada
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Quote:They do directly inject into the cylinder. They call it port injection. My 4.0L Jeep had six injectors, one for each cylinder. The injectors just don't inject at the top of the combustion cycle like a diesel does.
Tom
I thought that the port injection system still injected the fuel into the stream of air on its way to the cylinder before the intake valve.
Bert
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Cdog
Stillwater, OK , USA
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They still have not overcome the problems with direct injection for gas engines. MPI is what is in use today by all manufacturers and it injects into the airstream just above the intake valve. For some reason the CRI systems like the diesels use to put fuel directly into the cylinders, does not work well with gas engines. They have been unable to achieve reliable injector service. They claim 15-20% gas milage improvements with direct injection, but they have to make it work consistently first.
22' 73 Diamond Class C
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laudy
columbus, nebraska,usa
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While I towed with a gasser for years, I traded my gasser for a diesel. Someone in a previous post mentioned if the both a gasser and a diesel had the same horsepower the ease of pulling would be the same and understandingly so. However I have seen some gassers with greater horsepower than the diesel and could, granted, pull with ease,............. but I personally never seen any of "MY" gasser tow vehicles pull at the same miles per gallon that my diesel would pull at. All of my diesels easily got better m.p.g. than my gassers. Also while it might not mean anything to others, I personally like the fact that the diesel doesnt seem to be laboring as hard as all of my gassers did when pulling. Just my 2 cents worth. Good luck blt2ski!
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BertP
Edmonton, Alberta, Canada
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I saw an item on the news last night that I thought was applicable to this thread.
With all of the fighting in Iraq, a US Army Abrahms tank was disabled in Baghdad. They didn't say what had happened to it, but it appeared that it had hit a mine and blew the track off. Without the track, the tank was completely immobilized. So, the Army sent a tank recovery vehicle, which is basically another Abrahms tank with the armament removed and replaced by "tow truck" type stuff, to get it back to base. A tank recovery vehicle showed up and backed up to the tank. They then attached some chains from the recovery vehicle to the disabled tank and towed it back to base. As I watched it go down the road, it occurred to me that they had a 70 ton "tow truck" towing a 70 ton tank. There was a rig with a GCVW of 280,000 lb being propelled down a Baghdad street by an engine developing less than 300 lb-ft of torque. Amazing.
Cdog - Interesting info. Do you have any idea why they are having such a hard time getting it to work?
Bert
* This post was
edited 09/09/04 05:43pm by BertP *
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Aquaduct
Winchester, VA USA
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Quote:I am curious, though. Why can't you inject the fuel directly into the cumbusion chamber in a gasser the way you do in a diesel? Is there a problem with the way it would burn? I seem to remember reading that detonation is basicly a really messed up flame front instead of a smooth one. Would direct injection of a gasser cause that kind of a problem?
Yes you can. It's been a while since I really paid much attention to the gas side of the world, but I think that there are at least a couple GDI (Gasoline Direct Injection) systems on the road today. I'm thinking a couple Japanese makes and a couple super-luxury European cars. In fact I was reading about some ideas of using GDI to develop a gasoline heavy truck engine for an anticipated economic shift away from diesel in the next decade and a half.
Actually, the concept was fairly sucessful in the pre-carbeuretion days (1930?). There are a couple fundamental issues with it even before you get to reliability questions.
First, a diesel has to use compression ignition due to the higher flash point of the fuel, so it's not as simple as applying diesel principles to gasoline, although there are a number of parallels.
The advantage of current homogenous port injection is primarily one of speed. When you burn the mix, you try to get the most pressure rise out of the combustion as possible. So you initiate combustion just before TDC so the fire is really burning and creating pressure to push down on the piston as it rolls past TDC. The problem is that as the piston moves down, volume is increasing lowering pressure and temperature. So it becomes a race to create as much pressure as possible before the moving piston neutralizes it. Fuel only burns as a vapor, so if it's mixed and a vapor before entering the cylinder, it burns faster than having to diffuse, vaporize and then burn like in direct injection. Faster burns mean more power, which is the inherent strong suit of gasoline spark-ignition port-injection sytems.
(NOTE: Before all the diesel fans get all grumpy, I'm not saying gassers make more power. It's an inherent advantage of the combustion cycle which can be overcome with things like turbochargers, just like fuel economy is an inherent advantage of the diesel cycle.)
Port injection also runs at stoich which makes emissions easier to manage with TWCs.
Direct injection gives finer control over the cylinder mixture so you can create rich zones around the spark plug that will let you run leaner than port injection. Something like 40:1 as opposed to 17:1 for a port system which is where you'd start to lose your ability to ignite reliably. More precise mix control also lets you run at lower rpms (ie.- 600 idle vs 750) and the cooling effect of the direct injection of fuel in the cylinder lets you run higher compression ratios without knock. All this adds up to diesel-like fuel efficiency on regular gas.
However, the same emissions problems of diesels crop up. NOx is high and there can be HC problems is the mixing isn't right. So now you're into higher injection pressures, more EGR, and the ever-challenging NOx reduction in O2 rich environments.
Like I said, I believe it's out there. I think one system is even set up to run both ways. The intake ports come into the cylinder in such a way that they can inject in the compression cycle for GDI operation and fuel economy, and then inject in the intake cycle for homogenous mix and power. Something says Isuzu or Mitsubishi. They've also got a Lean NOx Catalyst (LNC). EGR is used to control knock at the higher homogenous compression ratio. I think at some point, you also have a problem with too much EGR which then requires a high energy plasma or laser ignition system. But I don't think that's part of the systems on the road.
By the way, did you ever get the note I sent you?
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