I've already heard the explanations of higher compression ratios making a more efficient engine. This is another theory I came up with.

First the assumptions.
1. Atmospheric pressure = 14psi
2. The engine pulls 20 inHg at 2000 rpm
3. The engine has a 4.00 inch bore and a 3.75 inch stroke


With a carburated engine, the engine will be pulling a vacuum in the intake manifold. The vacuum in the intake manifold will be the same on the surface of the piston during the intake stroke. If there is a vacuum on the face of the piston, then the atmosphere is exerting pressure on the back side of the piston. How much pressure?

Surface area of the piston = 3.14 * 4.00in = 50.24 square inches

If at 28inHG the atmosphere exerts 14psi then at 20inHg the atmospheric pressure would be 10psi

Then at 20inHg the atmosphere would exert 502.4 pounds of force on the backside of the piston

With a 3.75 inch stroke that is 157 ftlbs of torque.

At 2000rpm, the intake stroke consumes 59.787hp per cylinder.

Since a diesel engine doesn't have a carb or throttle plates, the vacuum seen by the piston is very close to 0inHg. A diesel doesn't experience the pumping loss that a throttled gasoline engine does.

So theoretically, if a gasoline engine could be made with fuel injected directly into the cylinders and no throttle blades, that power lost performing the intake stroke would be available to move the load.

 

The problem with running a gas engine wide open like that is the explosive nature of gasoline. The cylinder pressures would be too high, and cause damage to the piston, head, and block. Plus, since gasoline burns dry, the cylinder walls wouldn't get enough lubrication like they do with a diesel.
There are modern vehicles that use direct gas injection, but they inject it during the intake stroke.

 

great work there, 76supercab!

 

A big reason is the BTU difference. It's like 150K BTUs per gallon of diesel vs like 120 BTUs per gallon of gasoline. There's more energy per gallon, so the gallon goes further. That and the higher compression ratios are what makes diesels get better MPG than those on the same platform.

Mike

 

diesel is more efficient. try a google search.

 

Yep, it basicly comes down to more BTUs per gallon, higher compression ratios, direct fuel injection, and turbocharging.

Mike

 

Air fuel ratios also contribute. 20 to 1 under power and 60 to 1 at idle, compared to a gas motor's requirement of 14.7 to 1 (or close to it) at all times.

Hot Rod a couple of months ago had an article on oil burners. We are not getting the good ones here in the US. Look into the engines BMW and Mercedes have. These would be great for SUV's and half ton PU's.

 

so in theory, you just proved why 2 stroke motors make substantually more power than a 4 stroke. that is also why 2 stroke diesel are absolutely awesome other than they scream.

 

todays 2 stroke outboards are setup this way

 

consider a gas engine can be no more than 35% effcient.

 

I'm going to quote our own BigF350, as he bailed me out on another thread in another forum. The question related to moding diesels vs. gas engines, and why it was easier...and more efficient to gain HP out of a diesel than a gas motor:

" Efficiency, not only does a diesel engine work on the diesel thermodynamic cycle (as opposed to the Otto cycle for gas engines), which gives it a theoretical maximum efficiency of 63.2% (52.3% for the Otto cycle), it is also capable of "lean burning" as mentioned above. So, for a given amount of fuel injected - more power can be extracted.

Compression Ignition - means you don't have to worry about tuning ignition (or even providing ancillary power for it)

Fuel injected at TDC, you don't have to worry about pinging, or running on a high performance fuel. Where on a gas engined vehicle with a turbocharger or a supercharger it is constant comprimise - to run more boost you have to lower the compression ratio to prevent pre ignition (and its associated problems). You can run 150PSI with a 18:1 CR if you want (people do...), on fuel purchased at your local truck stop."

 

You beat me to it

Those are the therotical maximums though.
The higher compression ratio of diesels also help, not to mention turbocharging.

 

the mb diesels we do have running around here in the usa . take a look at the "yak" dodge/freightliner highboy vans. thats a 5 cyl. mb diesel. look for the long time here name of MTU. thats also mb but the ones i have seen and run. they are more known as a top of the line marine/big rig power unit motor. for more info check out the detriot/mtu web site."yes mb owns detroit"
Now direct gas injection thats some thing the world has been playing with for some time with 2 basic ideas 1 from down under that merc.marine uses " i forget the name big please help " uses in the out boards and the fict/ german unit used by omc.and later by a few car companys from the far east and germany, the one that comes to my mind as i have worked on them is vw with it's new 4 cyl., 2.0 liter direct injection, high compression, turbo,intercooled that pumps out 207 hp. "103.5 hp per 60something cubic inches " drive one in a passat or jetta gli just to get a idea for your self of how this sysytem works and runs for your self.
changes in to controled gas combustion are comeing faster then most of us would like to see. but to me the direct injection gas sysytems being used in outboard and automotive uses are going to change a lot of things. and how we see gasoline motors down the line.

 

That is a pretty impressive system - the VW system, I had a chance to drive a VW golf with the 2.0l turbo engine that you mentioned. It was impressive.
It was pioneered on the Audi R8 Le Mans car if I recall correctly???

The system Mercury uses in its Optimax outboard engines is interesting - Its a mechanical Orbital system, and they are 2-strokes....

 

the orbital started down under,fict is from germany 2500 psi at injector