compression and thermal efficiency in ICE 1

[svizoman]

I was watching Mazda's skyactive diesel tecnology promotional video. They claim that by lowering compression ratio to 14:1 they lowered emissions and also improved fuel efficiency. In regular diesels injection begins when piston is at round 35°TDC but with skyactive D injection starts at 0° TDC. The thing is that there are actualy three injections, pre, main and post injection and I don't see the reason why in regular diesels with 18:1 or 16:1 compresion ratio you should start injection later. And old VW TDIs in real life achieve better milegage than Mazda Skyactive.
The only reason to lower compresion is to reduce PM and NOx and to get smooth engine. Those engines can be of lighter materails achieving higher RPM 5000+ instead of 4000.

Mazda has the lowest compression and thus don't need expensive aftertreatment systems like DPF, NOx, H2S, Adblue integrations. But on my opinion engine can't be more thermal efficient than regular diesel, contrary I say it is less thermal efficient. They may gain fuel efficiency due to lowering friction loses, better fuel burning due to improved piston's cavity and new high pressure, high speed piezzo injectors but I think it is a lie about compression.

 

[gruntguru]

Efficiency gains by increasing compression ratio are really due to the increase in expansion ratio. A problem for diesel engines is a long combustion duration meaning much of the later heat release is expanded far less than the mechanical ratio. Although not stated explicitly it is suggested that the Mazda engine has a more rapid heat release. If so that could easily compensate for the small efficiency reduction resulting from a CR reduction from say 16 to 14.

je suis charlie

 

[BrianPetersen]

When the compression ratio goes up, heat transfer (losses) goes up, crevice volumes become a greater proportion of the total volume at TDC (impairs fuel/air mixing), friction goes up (more pressure on the rings against the cylinder wall), the weight of all of the parts involved goes up because of higher peak cylinder pressure.

I thought it was well known that raising the compression ratio had diminishing returns. There's no hard and fast line beyond which it starts getting worse but 13:1 - 14:1 is commonly quoted.

You can't compare old pre-emission-control diesels to anything with modern emission control. Skyactiv-D still has a DPF and still needs its regeneration strategy, and it still uses EGR for NOx control. Obviously Mazda was not able to reach compliance with North American emission standards with this approach (promises promises, but they ain't in the showrooms) and I can pretty much guarantee that they are using heavy application of EGR for NOx control, which is known to have a fuel consumption penalty compared to using SCR ... but then you don't need an SCR and the end user doesn't need to deal with it (they have had reliability problems).

The old pre-emission-control VW TDIs use less fuel than the VW common-rail TDI, too, and even at that point VW was cheating to get all they could out of the TDI.

I'm quite sure Mazda did their research. And didn't cheat.

 

[tbuelna]

The geometric CR of a recip piston engine is not really what matters. The more important thing is overall cycle pressure ratio. It is often more efficient if much of the gas compression/expansion process can be performed using turbo machinery. Using a CR of 14:1 in the recip piston section is the minimum practical limit for most commercial engines. A low CR gives the min peak combustion temps, which is critical for minimizing cylinder exhaust gas NOx emissions.

 

[svizoman]

{Efficiency gains by increasing compression ratio are really due to the increase in expansion ratio. A problem for diesel engines is a long combustion duration meaning much of the later heat release is expanded far less than the mechanical ratio. Although not stated explicitly it is suggested that the Mazda engine has a more rapid heat release. If so that could easily compensate for the small efficiency reduction resulting from a CR reduction from say 16 to 14.}

Let me check I get it right....

Old high compression ratio diesels get high compression temperatures on the end of compression stroke, thus when injector opens fuel molecules ignite right away, causing unefficient burn (PM and NOx particles are formed) and rapid change in cylinder pressure. By the time injector closes the piston is allready part of its way toward BDC and thus there are lot of pressure and temperature to be unused.

Since diesel fuel are long molecules with slow burn rate, lowering CR (temperature) you can apply advanced injection (before pistons reaches TDC) since it will take some time before molecules of fuel will ignite. Durring that time fuel will travel further in the cylinder, mixed with more volume of air, forming better atomization and when finaly it will ignite it will be more rapid release of energy thus more of the energy will be extracted and EGT will be lower meaning better thermal efficiency.

 

[gruntguru]

Diesel fuel actually burns quite quickly.
Fuel injected before ignition ("ignition delay") is actually a problem and causes "diesel knock". The Mazda engine avoids this problem by heating the induction air (including hot EGR).

je suis charlie