1. Diesel's were the first to use DI mainstream....What makes diesels a better candidate for DI than gas engines which have just recently been getting overhauled to use DI?

2. How does DI allow an increase in compression ratio?

3. Why can you have a 64:1 A/F ratio with DI?

4. What kind of A/F ratios do current direct port injection engines have?

 

Three-way cats require 14.7:1 if they're going to work. So current U.S. engines will run at that ratio, even the DI models, at least most of the time.

DI engines could run much leaner for a while, storing NOx, and then letting it out when running at 14.7, or they could use a lean NOx reduction technology like urea injection. NOx storage requires a special type of catalyst too.

My understanding is that very lean A/F ratios result in lower combustion temperatures, which reduces engine-out NOx, so that in countries with easier NOx standards, they may be able to run lean all the time, or most of the time. But in the U.S. you need aftertreatment.

 

A lean A/F ratio will increase NOx realitive to a richer ratio.

1. Don't remember, but the I think one of the reasons that DI has just recently been making it to gas engines it because port injection has been able to do a great job of meeting emission standards with providing good gas milage. There is an increase in control of these with DI, but the just isn't as big as it was from carbs to EFI.

2. The fuel enters the chambers at much cooler temps and can better cool the cylinder temps.

3. Since the fuel is entering the chamber and does not sit behind the intake valve, more fuel enters the cylinder at the higher A/F ratios allowing it to run at high A/F ratios. I'm not sure on 64:1 on gas engines, but I know for a fact the turbo 2.0L in the Sky can run as lean as 32:1.

4. The Sky is the only one I know of as fact. Not sure any ony other systems.

 

1. Diesels *have* to use direct injection. Their combustion timing relies on the fuel spontaneously igniting when injected into the superhot compressed air inside the cylinder. That's why Diesels are categorized as "compression ignition" engines as opposed to a gasoline's "spark ignition". Note that some Diesels use a small pre-chamber to set off the ignition, but these are still "direct" in the sense that fuel is directed straight into the combustion chamber as opposed to being mixed with the intake charge.

For gasoline engines, direct injection is a lot more complicated than a carb or port injection (especially when you have to deal with emissions), which is why it's been slow to adopt. Diesels have no choice but to use direct injection.

2. Mostly because the spray of the gasoline directly into the compressed air drops its temperature significantly as it evaporates, and a cooler charge is less prone to detonation so they can up the compression ratio compared to a port injected motor.

3-4. I don't know what A/F ratios DI engines run, but the thing to remember is that the mixture has to be close to stoichiometric near the vicinity of the spark plug or you won't get reliable ignition. So the injector is aimed pretty close to the spark plug and that little pocket of air is more or less normal richness, but the air in the remainder of the cylinder is way lean.

 

I remember looking at a techdemo on an official Honda site a while ago, and it explained under cruising/light load conditions, they envisioned only shooting a bit of fuel into the very center of the piston at the last possible instant of the COMPRESSION cycle and then immediatly ignite just that...

Really crazy stuff!

is that what they really do now?

 

From articles I've read about DI, thats about exactly how it goes down. Also, the piston has a dished out portion on the top to help localize the burn. Probably to keep the lean A/F mixture in line.



Cool stuff.

 

thread 1

thread 2

thread 3

Here are three threads from the Adv section that talk some about DI.

 

[QUOTE=mastrdrver;4345878]A lean A/F ratio will increase NOx realitive to a richer ratio.
/QUOTE]

Are you sure? I understood that a lean A/F ratio would reduce NOx. However, it also renders ineffective the standard catalytic converter for the purposes of NOx reduction. However, the lower combustion temperatures of lean combustion should result in less engine-out NOx.

The various NOx adsorbers will allow NOx to be stored for a while and then reduced by mixing with ammonia. Honda has a new catalyst design which will automatically generate ammonia by briefly running rich. That means the Honda diesel engines won't require a urea tank like the Mercedes diesels. Up to now, I believe GM was also planning on the urea injection. If the Honda invention actually works, I wonder if they'll license the technology out?

http://en.wikipedia.org/wiki/NOx_Adsorbers

In case you confuse adsorb with absorb:

http://en.wikipedia.org/wiki/Adsorb

 

Lean burning causes elevated combustion temperatures and the resulting NOx emissions. Running lean does in fact result in higher, not lower, temperatures.

Running rich results in lower combustion and exhaust gas temperatures but increases CO and hydrocarbon emissions.

The standard gasoline engine catalytic converter does not adsorb. It reduces and oxidizes in different stages. The reduction stage breaks down NOx into free nitrogen and oxygen. The oxidation stage uses free oxygen to oxidize CO and hydrocarbons resulting in CO2 and H2O.

In normal operation the engine crosses rapidly between rich and lean right around the stoichiometric point in order to maintain catalytic converter function.

Keep in mind that although DI engines can run "lean" it's a relative lean compared to the entire cylinders' fuel and air charge. The whole idea behind DI is to have so much control over fuel delivery that you can create a stratified charge and maintain a specific AFR within a localized region. The "actual" mixture seen by the flame front is not lean.

 

That's a good clarification.

 

That's the case for slightly lean (16:1, for example), but what about much leaner ratios that you get in so-called lean burn engines?

Right. The adsorbing converters are what they're planning to use in lean burn engines, which all diesels are. It's more expensive and less effective, but it works with an oxygen surplus.

 

I don't think it's quite that simple which is where my last point comes into play. The exhaust gasses are seen as that lean but the flame front doesn't see a mixture that lean. There simply wouldn't be a flame front in a gasoline engine with that lean of a mixture!

The combustion would be very hot however some of that thermal energy would be absorbed by the remaining air charge. I don't know offhand if the resulting exhaust gasses are hotter or cooler and I'd assume it matters a great deal on the exact conditions in question.

I guess you could think of it almost like EGR except you don't need inert exhaust gasses since the localized mixture is controlled so well.

We're talking about exhaust gasses though and the original topic was NOx emissions which are created during the combustion process. If talking about NOx we need to look at the pressures and temperatures that the localized mixture sees. It isn't going to be 64:1

 

Fair enough. I just recall reading quite some time ago that lean burn engines produced less engine-out NOx. For example, there were quite a few sold in Japan in the 90s that met their NOx standard without any NOx after treatment. Whereas the stoichiometric cars all used a 3-way cat. Honda even had a 49-state Civic in the early 90s that ran lean burn much of the time (for California, they had to use regular stiochiometic).

But I'm no combustion expert. I just read stuff

 

Remember that the oxygen sensors that can actually tell you a A/F ratio is taking sampling 4x per second. Plus, what you see on a hand held unit in just an average that updates every few seconds. What you get is not the acutal A/F ratio of one cylinder, but the average of all the cylinders it is measuring.

Remember also that the EGR cuts down on the NOx the engine puts out. It also cools the cylinder by adding NOx back into the engine.

 

There isn't much NOx to add back in. I thought the main cooling effect was from all the inert gas (mostly CO2) that dilutes the mixture.