treyZ28

In theory, having an instantious pressure spike at 90* ATDC would be the best place to apply the force, correct?

But other problems come into play- especially having a compression ratio of about 2:1 and a definate need to ignite the fuel before TDC when it will combust on its own due to pressure/temp

So here is a big question, with direct injection of spark ignited engines- is anyone doing this.

set up an extremely high compression engine so that at say 60* ATDC, the compression ratio is about 10:1. (sorry if it doesn't make sense, its hard to explain on the net) and inject the fuel at that point and then ignite it. This way peak pressure is between 80* and 90* ATDC.

I know people have tried variable compression heads so the head follows the piston without prevailing- just wondering what is going on with this

Adrenaline_Z

If you are referring to LPP (location of peak pressure), it is 14 degrees ATDC.

An instantaneous BOOM at 90 degrees might be best in lever theory, but
that instant spike isn't going to yield much duration of pressure.

That leaves alot of work for the other cylinders and flywheel to take up the
slack.

You typically want combustion to start early and expand within the volume
between the piston crown and cylinder head.

As the mixture expands, it continues to apply pressure to the piston crown.

Injecting the mixture at 90 degrees would require large amounts of charge
to get the same pressure as the volume in the cylinder is much greater.

I'd hate to be the poor sucker to grind that cam and make it work ;P

treyZ28

Like i said, in theory. obviously, the most force*lever arm under the curve is ideal.

mabe i should say "delaying the combustion process and creating equal compression ratio about 70 degrees later"

Adrenaline_Z

I think there's much more to think about.

The distance of the piston to the crank centerline (and rod angle),
EVO - giving up 70+ degrees of combustion force.

I can't see this concept working in the real world, or on paper.

RoAdRaGe912

by waiting to apply full force at the peak leverage angle like that you'd be wasting a lot by letting the crank spin instead of applying force to push it down.

yes 90 degrees would be the best leverage angle but by doing it your way instead of exerting at least SOME force before 90 degrees ATDC you are doing exactly nothing for those degrees of rotation.

it would be MUCH less efficient. to even match the power of the traditional way you'd have to be burning more air/fuel to have a higher than normal pressure than you would have needed at 90 degrees ATDC in the traditional setup.

basically just a big step backwards for efficiency.

'JustDreamin'

I find direct injection to be a very interesting topic.

Its made even more so now that there have been production engines running direct injection for a couple of years (Mercury & Evinrude / Johnson Outboards). Granted they're 2 strokes, so some things don't compare directly, but they've still done some great R & D in order to make it work. I just haven't seen alot of information floating around on it. Anybody got any good links?


Ultimately, power output is all about maximizing pressure under the curve during the power stroke (duh), while minimizing the losses incurred in the compression stroke. Think about the way a steam engine works. They inject high pressure high temperature steam just about TDC, and it expands through most all of the stroke. All about getting the most energy (temperature & pressure) out of the working fluid (steam or fuel).

I'd be real interested as to what the injection timing on the Outboard motors mentioned above is. And compare that to the way diesels do fuel injection. With good enough injector technology, I think you would want to do a couple of pulses, one (or more) on the compression stroke to cool the charge off, a small pulse to get combustion started, a main pulse with the bulk of the fuel, and possibly even a pulse during the power stroke (that one is questionable, and only possible if there is excess oxygen available in the cylinder).

'JustDreamin'

BP

The new Mazdaspeed 6 is advertised as having direct gasoline injection.

bowtieman81

iTrader: (35)

There are actually direct injection cars outside the U.S. Search for a Mitsubishi DI engine. They are producing them for Asian markets. Did a report on them back in college. Found lots of good info.

CashMoney

iTrader: (2)

http://world.honda.com/HDTV/news/2003-4031127a/

http://world.honda.com/HDTV/news/200...05b/index.html

ASRoff

Some of the early aviation radial engines were direct gasoline injection but the systems were so complex at the time that it wasent long before it was abandoned.

chuntington101

Mitsubishi have been running it for over 7 years now. but they are using spark indused combustion! its is supossed to be a good idea because effectivly you take the injector and the fuel "spray" out the air streem, letting you get more air into the cylinder and thus more power.

as for running a gas engine like a deisel, i think Lotus are looking into. i heard about them building a super High comp. engine a few years back that would run on any thing! not heard anything for ages though!

as for the best place to "light/inject" the fuel, you want it at the lowest speed/max acceleration point! this is so you have a longer time period, and a smaller volume (thus higher presure for any given volume of products), to exsert the forces on the piston. this place is at TDC!

thanks Chris.

white2001s10

iTrader: (4)

I think of another problem.
To develop a significant pressure spike inside the usable area @90*ATDC the amount of energy would have to be tremendous. The most exposed surface area for the pressure to act on would be the cylinder walls and not primarily the piston. I don't think standard cylinder walls would be strong enough to contain that much energy exerted over that surface area.
The standard 4-stroke gasoline engine probably couldn't handle this kind of operation, but there may be hope for deisel or 2-stroke if the cylinders are strong enough.

1FASTS10

Audi is another one that is now implementing Direct Injection, very nice stuff!

Ric

Largely because I'm an idiot who likes to think he knows something, but I kinda envision a knock tendency with a DI gas engine. Wait a minute, no I don't... with the proper chamber design, and possibly multiple charges(sort of a booster charge, then the main charge) like JustDreamin said, it could happen.

Far as 90* ATDC being the optimum energy transfer point, that's true... but then, how does the piston get there??? The 4-stroke piston engine has each piston under combustion for 180*, with that 90* ATDC right in the middle. One seriously limiting factor in making power on a gas engine is... well, the gas itself. I know we'll probably never see the world convert to nitromethane, but let's compare the two. Gasoline actually has a higher peak pressure than nitro, because gas burns almost instantly, then as the piston is forced down, it has less pressure operating on it. Nitro, on the other hand, has a much higher average cylinder pressure, because it burns so stinkin' slow. Yet it creates much more energy than gas, too... how else could a 7000-hp Fueler go from a 2300-rpm idle to 8500 rpm in less than a tenth of a second?

Maybe we just need to convince the rest of the world that nitromethane is safe, efficient, and economical.

P Mack

iTrader: (6)

If there was such a thing as instantaneous combustion you'd get the most work out of it combusting at top dead center, letting the pressure push down on the piston the full length of the stroke.

chuntington101

Dam, forgot about them! the VWA group is using DI on lots of its engines now, and i have heard rave reviews about most of them.

i think someone needs to comition some chemits to try and find a fuel that combinds all the good things of meth, nitro and normal gas to make a safe reiable efficent and powerfull fuel. it must be possiable its just all the big gas companies push them under the carpet!

thanks Chris.

RoAdRaGe912

well, if there were such a thing as instantaneous combustion, then that would also mean to instantaneous pressure correct?? you would not want to apply that pressure directly at TDC. if you did so, it would not be applying the force in the direction of rotation, but instead straight down. you would want to have that instantaneous combustion slightly after TDC so that the work could be applied to rotate the crank instead of trying to force it out of the block.

P Mack

iTrader: (6)

By waiting until 90* ATDC you would have better leverage on the crank, but also much less cylinder pressure because you've let the charge expand to a much larger volume. And even waiting to have better leverage doesn't help you because if you ignite at TDC, you will eventually end up with the same cylinder pressure and leverage once the crank gets around to 90* ATDC. In other words, you'd have a net gain of 0 from leverage, and a net loss of half the energy available by igniting halfway down the pistons travel.

As far as forcing the crank out of the block, maybe the stresses on the bearings or mains would rule it out, but as far as thermodynamics go, instantaneous combustion ATDC would give you the most power.

P Mack

iTrader: (6)

I think the reason everyone is thinking 90* ATDC is cause the original poster said "instantaneous pressure spike" implying that the pressure is there for an instant then gone the next instant. Cylinder pressure vs time would look something like this: _____|______ the way it was originally asked

When it should be _____|''''''''''''''''

It's not like all the cylinder pressure from combustion at TDC would instantly be gone at 1* ATDC.

Sorry for the crappy drawings, it's the best i could do with text characters.

RoAdRaGe912

i only mentioned it being forced out of the block to try to convey the idea of the force ONLY be exerted in a doward direction. that force would not help to rotate the crank at all and could possibly slow down the rotation.

i'm not sure who is stuck on 90 degrees, but all i was saying is that it would be better to apply the force ATDC instead of directly at TDC. by ATDC i mean slightly.