Some may have already addressed these issue, but here’s my 2¢.
DI gasoline allows higher compression ratios by utilizing the heat of compression in the cylinder to further vaporize the fuel. By absorbing that heat, it does in fact lower the actual compression pressure, along with the temp (remember: PV=NRT).
But the prime selling point for DI was initially that it allowed for stratified layering of the fuel; rich at the plug, lean elsewhere. Problem is that results in high lean NOx emissions, which don’t fly in the US, but are allowed in Europe. That’s why DI got popular in Europe first, less so here. So when DI came here, they didn’t utilize the stratifying technology (at least at first. I’m not familiar with what Mazda has since done) and thus fell back on the charge cooling advantage as its selling point.
As for injected droplets verse vapor: they are the same thing. What vapor is is air with very fine droplets. So it’s not an either/or.
Not sure I get your last point. When gas evaporates it is no longer in droplets, but in individual molecules floating around.
I actually ran a regular push mower engine on Coleman camping fuel one time and it worked better than I expected. I had been busy working all day and ran out of gas not far from the end of finishing the yard. I know the Coleman fuel is more expensive than regular gas but I only needed a small amount and was frustrated and didn’t want to have to drive to a gas station. I put in a small amount in case it didn’t run so I could dilute it with regular gas the next day. It actually ran pretty well but might have had reduced power when pushing through taller grass that bogged down the engine. It wasn’t a good mower anyway and I had two so I didn’t care but it appeared to cause no long term damage. I don’t know if the results would be different if you did this for an entire mowing season.
Evaporated gas is still droplets. They are just smaller size than what we normally think of as droplets.
Boil a pan of water on your stove. The water evaporates, right? But if you look, and the room is cool enough, you see the water raising out of the pan as a vapor, which is a cloud of small droplets.
If the liquid has evaporated and is in a gaseous state, then it is individual molecules and is invisible. The “steam” that you see rising out of the pan is water that is recondensing in the air and is visible again. It would be small droplets.
Yeah, it was a poor analogy on my part.
My point was that most of what people think of as fuel “vapor” is simply very well atomized droplets.
That’s incorrect. The majority of fuel components go into the vapor phase. Some of the very heaviest may remain in a droplet, but they vaporize during the combustion cycle.
No, vapor =/= droplets. Vapor is humidity; droplets are fog. (I.e. solution vs suspension of a liquid in a gas.)
Think of it this way - take a pie pan, put a tablespoon of gasoline in it (outdoors, of course), come back it 10 minutes. The gasoline is gone, it evaporated, turned to vapor, not small droplets.
Yeah but gas doesn’t have 10 minutes inside the cylinder!
Hey–I’m not saying the current system doesn’t work. I’m not even saying it’d be “worth it.” I was just wondering if pre-vape’d gas would burn cleaner, is all.
Meanjoe, here’s the 1930’s version of your idea:
http://fuel-efficient-vehicles.org/energy-news/?page_id=986
Pre-vaporizing fuel has been tried almost continually since then; none have stood the test of time.
Great posts guys. I’m getting a lot from this input. Keep ‘em comin’.
I use the terms “droplets” and “vapor” to present a different mental image. “Droplets” sounds bigger than “vapor”. Truth is, it’s about the surface area per volume of the fuel, being as only those molecules in contact with the oxygen molecules tear themselves apart and bond to the oxygen, so the whole thing burns though in layers like an onion. The smaller the droplets of fuel (the finer the spray), the more thoroughly and rapidly the gas burns, meaning more complete combustion and better use of the energy in the hydrocarbon molecules. Thus, cleaner burn and better mileage for a given engine.
The whole issue of the chamber temperature and gas density is what I thought DI might be doing, in addition to eliminating the inherent disadvantages of having to pull the vaporized fuel past the intake valves. But it’s a subject I’m still learning about, and it’s good to have more of the “what actually happens” information. Lots of old physics images from old textbooks is appearing in my brain. And I love it.
Mark, IMHO all the cars you’ve mentioned are great cars. Much of what separates them is basically just taste.
It’s nice to give people some options to check out. Some good cars just don’t personally appeal to me, and I’m sure the same is true of others Some people like plush interiors, some want to clean them out with a hose. Right now the mid-sized models are almost all good, and small crossovers mostly very good, too. When people ask for recommendations in those categories I try to remember which cars CR has been condemning for poor reliability, like most recent Fords. I wouldn’t even have a problem recommending one of them to someone buying a low-end model with a manual transmission and without Sync. Almost all Ford’s problems have been in the dual-clutch transmission and Sync (which they’re throwing in the towel on.)
I go to the SF Intl. Auto Show every year (since I moved here in 1987) and every year the cars have gotten nicer. It used to be about half the new models were off in some way (and I don’t mean to drive.) The seats were terrible, the ergonomics unfathomable, the plastics shiny and ill-fitting. Now I can only think of a few cars like that, and they tend to be the very cheapest models and typically designed mainly for other markets with greater price sensitivity. Like the Nissan Versa. Even those cars are usually fairly reliable and I’m happy people can still get into a new car for under $15,000 - with rebates and whatnot, sometimes quite a bit less.
In 1981 or so, my parents needed a new small car, and even then the cheapest fairly nice ones were close to $10,000, and they had fewer features than even the cheapest cars now. They ended up with a Renault Alliance that was cute, surprisingly roomy, and had great seats, but was slow, impossibly unreliable, and couldn’t be driven up into the mountains without sounding like it was having an asthma attack. Such a lovely car when it ran. The inflation in cars has mostly been in the upper half of the market, and those cars have luxury and performance almost unimaginable in 1980. These are such good days to be a car buyer, especially for those of us who lived through the seventies and eighties. I’m not a mechanic, but those of you who had to work on some of those cars have my deepest sympathy. At least they kept you busy.