Hacker News Reader

Comment by ajross | Hacker News Reader

Gah, terribly confusing terminology. This is a Diesel engine. "Compression ignition" is what the Diesel cycle is all about. It's just carefully tuned such that it can run on gasoline instead of kerosene (i.e. "diesel fuel").

And mostly I don't get it. This is just a convenience factor. If you want a high-efficiency diesel right now you can buy one already. Low-volatility fuels are already more energy-dense (i.e. fewer losses carrying the stuff around in the tank) and require less energy to refine. This gives you the advantage of being able to pull into a gas station without diesel available, but otherwise doesn't seem to add much.

View on HN

It's not exactly the same as a diesel engine. Diesels use a stratified charge (SCCI), meaning fuel is injected at the end of the compression stroke and burns immediately, without having a chance to mix thoroughly with the air. This engine uses a homogeneous charge (HCCI), meaning fuel is injected at the beginning of the intake stroke but does not ignite until the end of the compression stroke, therefore having time to mix with the air. This subtle difference makes the engine much harder to control, but should yield lower NOx and particulate emissions as well as higher power output.

Also, the engine is capable of running in HCCI mode under light-ish load. It still has spark plugs, and can run as a normal gasoline engine (HCSI) when high power is demanded.

This is much more interesting than that. It's a gasoline engine (fuel injected at intake and spark to ignite) that can operate in compression ignition in some of its range and thus get better mileage[1]. It should get efficiencies much similar to diesel but me much cleaner (less NOx).

[1] https://en.wikipedia.org/wiki/Homogeneous_charge_compression...

Taken from the article linked above: https://www.reuters.com/article/us-mazda-strategy-idUSKBN1AO...

> Its fuel economy potentially matches that of a diesel engine without high emissions of nitrogen oxides or sooty particulates.

Yes, however common diesel fuels tend to be higher in NOx and particulates.

My understanding is that because gasoline is refined to a higher degree it burns a fair bit cleaner in similar environments(and also makes it a PITA to store for more than a few months).

[edit]

We've got a pair of tractors around the house here, '81 diesel(pre-emissions), and '47 gasser.

I can tell you which one I prefer to run from an exhaust perspective, the diesel may have torque and is a dead-simple engine but you don't want to be downwind of it when cutting heavy brush.

NOx emissions from a diesel engine are from atmospheric nitrogen and oxygen, not from the fuel. I welcome further clarifications, but my understanding is that this engine will not produce NOx like a standard diesel because the fuel/air mix will be tightly controlled like in a gasoline engine, and temperature and pressure will be significantly less than a standard diesel. Additionally, it seems the timing of the fuel injection will be much earlier than a diesel, resulting in more complete air/fuel mixture; this is possible because gasoline has a higher ignition temperature, so fuel can be injected early and ignition won't happen until the compression stroke; diesel fuel would ignite during injection regardless of where in the cycle.

I'm struggling with the same reasoning: NOx is the result of high combustion temperatures, which is a pure function of the compression ratio (and fuel mixture), which is the thing that defines the high efficiencies you can get with a diesel cycle.

The argument in the linked wikipedia entry is that combustion temperatures are lower in this cycle because the better fuel/air mixing means it can burn much leaner and thus at a lower temperature. But a leaner mixture means lower power for a given displacement too, which means lower efficiency than a comparable traditional diesel (which are already hard-pressed to see gains like the 40% claimed against well-tuned gas engines).

Honestly the whole thing sounds very snake oily to me. I don't deny that it's possible such a thing could be tuned to operate as well as a traditional engine, but... it sounds awfully fiddly. I'd want to see numbers from a production engine in a real car before placing any bets. Electric continues to look like a much better bet to my eyes.

I don't buy the wikipedia argument either, but I do believe the homogenous charge mixture will burn at a lower temperature for a given compression ratio than the equivalent stratified charge.

Throwing f/a into the mix, the HCCI mode is apparently only active during low-power cruise, so the lean mixture is fine. The Mazda innovation is controlling the changeover from HCCI to SI when more power is demanded.

According to wikipedia the fuel can be a large source of nitrogen for NOx: https://en.wikipedia.org/wiki/NOx#Fuel

Diesel seems to be very much on the way out, with several cities discussing diesel bans. If I was to buy a new car today I certainly wouldn't consider a diesel (as opposed to two or three years ago when diesel was the future)

Reply on news.ycombinator.com