Back in the 1950’s, Popular Science had an episode every month about Gus Wilson, the fictional master mechanic of the Model Garage. In one of these stories, Gus helped a military man win a contest for waterproofing an engine by converting a Jeep gasoline engine so that it required no ignition system except for start-up. Gus accomplished this by drilling a hole in each exhaust valve face so that when the carbon built up and got hot, the engine would run by igniting the gas/air charge from the hot carbon in the exhaust valve. The engine would be started on the regular ignition system, and then after the engine warmed up, the electrical ignition system would be switched off and the engine would continue to run as a diesel. According to this story, the military had toyed around with the idea, but it didn’t really work very well because there was no way to time the engine. However, old Gus was able to help his military friend win the contest for an angine that would perform under water (there were stacks for air intake and exhaust). These Model Garage stories were, of course, fiction but were supposedly based on fact.
These Model Garage tales appeared in Popular Science from sometime in the mid-1920’s to 1970 and the author’s pen name was Martin Bunn. Somewhere I reacall reading that there were three such writers that assumed the name Martin Bunn in this time period. I found the stories about Gus Wilson and his feats quite entertaining and they did seem plausible.
I was under the impression that it was differential taxation of diesel vs. gasoline that resulted in diesel-happy EU, not so much the effeciency gains. In excange for the marginal improvement in power-specific fuel consumption, you’re stuck with soot and other not-so-fun stuff fouling up the air. The town I live in switched all the buses over to CNG from diesel for environmental reasons.
I think it’s some of both, diesels are definitely more efficient but the relative cost a gasoline/diesel can influence the market. As for as emissions go, it depends what you are looking at. Diesels are generally worse for particulate and NOX, but they are better for CO2. As always, no free lunch.
In a gasoline engine, the combustion is essentially instantaneous,
I don’t think so. I’ve seen the flames coming out the headers on gas powered race cars. If the combustion was instantaneous, you would want the ignition to occur at about 30? ATDC, not as much as 24?BTDC. You want maximum pressure to build up between 30? and 150? ATDC. At 150? ATDC, you want the exhaust valve to open to start venting the exhaust gasses from the cylinder. The power stroke is not TCD to 180? ATDC, but 30? to 150? ATDC. There is almost no power developed in the first 30? or last 30? of the cycle.
The flame travels very fast, but the individual droplets burn from the outside in, building pressure and expanding even after the exhaust valve opens.
Diesels run on jet fuel which would be pure kerosene if it could be counted on to stay lit way up there. Our planes used to use a naptha based fuel. The pilots used to call the throttles “kerosene handles”. We ran our diesel generators on the jet fuel after the first three tankfuls. Gasoline is too volatile and can’t stand the heat near the cylinders and would combust too violently if sprayed directly into a hot cylinder as diesel fuel is. That is what causes the knocking sound. 22 to 1 compression ratios will do that.
Toyota has direct injection gas engines,and I believe Ford will market an upmarket car with this as well. The injection part does not make a car a diesel; it is the self-ingnition that makes it a Diesel. Craig’s web image of the Mercedes cross-breeding the Otto and Diesel cycle will lead to some very advanced gas engines.
From the previous posts, you can conclude that: 1) diesel fuel is flame speed limited, so very high speeds are out of the question, 2) also gasoline engines can be built with stratifiedd or timed injection to allow very high compression ratios without high knock combustion. 3) Because of the make up of a barrel of crude, we will always have several fuels produced; light, middle distillates, and heavier residual for very large diesels and power boilers.
Good point. I had not thought of that. A gasoline line going directly into the cylinder head would likely be a vapor lock nightmare. That’s probably the best argument so far.
Perhaps if there were a recirculating system so the fuel doesn’t linger in the high temp areas. Race cars sometimes have a fuel chilling system and that could be a solution. This is all getting very messy, a whole refrigerant system just for fuel. But the gains might be worth it.
The 300SL Mercedes sports car of the late fifties/early sixties had direct injection with gasoline. I do not recall any vapor lock problems; don’t forget, the fuel is under extreme pressure prior to injection, and does not get a chnce to vaporize. The only problem with the 300SL system was post shut-off leakdown of fuel into the cylinders which caused black smoke on startup. The part of the injection line going through the head could be insulated, if necessary.
I remember in the operator’s manual for a GM 671 diesel (2 cycle blower scavanged 6 cylinder engine) powered bus that in an emergency gasoline could be used to run the engine. A ratio of lubricating oil had to be added to help the gasoline ignite quickly at the compression temperature and to lubricate the unit injectors. In other study, I learned that the precision lapped fitting of the plunger to the barrel in the injector will gall without some sort of lubrication be it diesel or oil/gasoline mixture.
Gasoline will knock more than diesel as it is designed to resist self ignition. Octane rating and cetane rating (gasoline and diesel respectively) are opposed, i.e. a high octane rated fuel has a low cetane rating. So with gasoline more unburned fuel will have been injected before combustion begins increasing the likelyhood of a detonation event. Use of gasoline might be more driveable and quiet in a prechamber/glow plug compression ignition engine rather than a direct injection engine.
The efficiency of a diesel engine derives from its nonthrottled air intake. Most of the energy taken to compress the air returns on expansion. So only a small amount of heat energy needs to be added to produce positive power output. The problem with a gasoline engine is that most of the time it is operating in a throttled mode with means that the mixture enters the cylinder at less than atmospheric. When that is compressed and burned, it expands at a lower pressure. This extracts less energy from the fuel burned. At full throttle an Otto cycle engine approaches the efficiency of a Diesel cycle engine. But as previous stated the Otto cycle engine spends little time at full throttle. Just going from the specific fuel consumption of gasoline engines, a 100-200 horsepower engine may only be cruising at 40-60 horsepower. A Diesel engine is also putting out similar horse power but the specific fuel comsumption is lower. The Diesel engine really shines over a gasoline engine at idle where it may consume 1/25th or so of the fuel it would at full injection while a gasoline engine consumes 1/6th of the fuel used at full throttle. Also in a gasoline engine the flame front has to burn all the available mixture for low emissions while in a diesel engine the air has to combine with all available fuel for low emissions (soot free) combustion. A Diesel cycle engine has to run with excess air to meet emissions while a Otto cycle engine runs close to stociometeric for low emmissions.
A problem with trying to use high compression to ignite a Otto cycle is that the ignition is only predictable at one throttle position. Getting the compression ignition to work from idle to full throttle would cause misfires at idle and detonation at full throttle. Direct injection and stratified charge might get around it. At this point of complexity, the diesel designed engine is actually simpler.
Well put! You have alluded to a number of reasons why industry and commercial operators strongly prefer diesel engines over gasoline. Many years ago I sold diesel engines and always had to explain to new buyers why an expensive diesel was a better long term economic proposition than a cheap gasoline engine. With or without excess air, or turbo charging, a high compression diesel will be more fuel efficient (lbs/brake hp.hour)than a lower compression gasoline engine. Many confuse this specific fuel efficiency with output per unit displacement, where a gasoline engine normally wins. Formulas 1 cars produce dizzy outputs per liter, but with atrocious fuel efficiency.
I meant “essentially instantaneous” with regard to generating pressure in the cylinder. The thermodynamic model of an otto cycle is that the pressure increase occurs very quickly near TDC (i.e., a constant volume process). Of course the is some combustion during the power stroke, there is some fuel that never burns in a racing engine, but it is not significant to the cycle. By contrast a dual standard air cycle (modern diesels don’t really run a diesel cycle), have some of the combustion during the power stroke (but not all). See the attached P-V diagrams.
You’re not interpreting the chart correctly. You need to understand the change in volume as it related to time in a combustion chamber. Near the top of the stroke, there is very little change in the volume of the combustion chamber. The change is almost insignificant from 30? BTDC to 30? ATDC. To see this, draw a circle. Draw a line from top to bottom. Now draw two lines angled + and - 30? from vertical through the center. Now draw a horizontal line through the points where these two lines intersect the circle. See how little distance there is from where the vertical line intersects the top of the circle and were the horizontal line intersects with it. Even though the crankshaft has gone through a +30? to - 30? arc, the piston hasn’t moved all that much.
At the point of ignition, just before TDC, there is expansion of gasses at the same time the piston is still going up, this will cause a spike in pressure. As the piston starts going down, the pressure drops at its fastest rate. Where the pressure curve hits the linear section, pressure is still decreasing, but not as fast, but at this point, you are in the -30? to -150? part of the stroke where the cylinders volume is increasing very rapidly, indicating that burning is still occurring and that gasses are still expanding. In fact, the gasses are expanding much faster than they were just after the point of ignition, indicating that combustion is still occurring.
OK, how about a spark augmented direct fuel injection. At one point I was thinking a full time glow plug would solve any issues resulting from a wide range of combustion parameters but really, as long as we?re sticking another device into the combustion chamber, just make it a spark plug. With air/fuel ratios severely lean of stoichiometric, a glow plug may be inadequate. A prechamber may be desirable.
High compression is really the goal we?re after in order to get gasoline burn efficiency beyond current compression limitations. I have to believe that modern technology solved any problems with gasoline injectors (i.e. galling) when leaded gas went away.
The standard three way catalytic converter that cleans up the exhaust emissions only work with a engine that run at stoichiometric to slightly lean.
The Japanese in the 80s and early 90s were hard at work developing lean burn technology, but strict new emission standards put a end to it along with two stroke technology.
The Japanese in the 80s and early 90s were hard at work developing lean burn technology
Tell you the truth, what they have developed is that the cheap way to make a system (mostly efficient injectors developed by its subsidiaries), basic studies have already been done by several manufacturers in Europe a long time ago. It’s just one of the mimic campaigns they’ve been doing.
I do think I can understand a P-V diagram, really. (-;
On this otto diagram, the combustion takes place between 2 and 3 at constant volume (essentially instantaneous at TDC, because that is the only time the volume is nearly constant). Between 3 and 4 the power stroke is modeled as adiabatic, so there is obviously no combustion going on (that’s basically what adiabatic means). What you are seeing during the power stroke is a expansion of the gas as the volume increases. If you looked at the corresponding T-S diagram, you would see the temperature is decreasing while the entropy was essentially constant (again, no combustion). Understand that this is an ideal otto cycle, and what goes on in an actual engine is much more complex.
I agree that the combustion in a real engine is not instantaneous, it actually takes place from some point BTDC to some point ATDC, but there is very little meaningful contribution to the pressure during the power stroke (i.e., it is close to adiabatic).
It not only will work but does in industrial engines. The head is modified to lower the compression and accomodate natural aspiration. Then the intake manifold adapted to accomodate an lp gas system and voila(That’s voyla here in the south) it does just fine. These are often used for forklifts and other industrial trucks where weight is needed as a counterbalancer. I rebuilt one a few years ago that was from the '80 s