Mazda 3 skyactiv Opinions

[MattBimmer]

Hey all,
Looking at getting a Mazda 3 with the Skyactiv and I am curious as you guys opinions on Direct Injection motors. Do you think this motor is gonna be a total pain in the A due to carbon buildup on the valves or do you think Mazda has made enough mods to the PCV from experience with the SPEED lines (direct injection). Should this kinda issue scare me or is it the way to go. Also does anyone know of a catch can installation on the new skyactiv motor?
Thanks ahead of time

[Dj Buttchug]

skyactive engines are pretty much the shit. i dont know about a catch can setup but im sure you could install one fairly easily

[MattBimmer]

Ahh shit got a response from the buttchugger. My Life is complete.
Lol, alright cool thanks for the response. You think a regular maintenance of seafoam would help keep carbon build up to a minimum?

[bmwstephen]

i'm more or less concerned with the cost of replacement items for all these bits of technologies they are cramming into the engine to max out fuel economy. A+ for effort but it's still too early to see how reliable they are.

I recall that before the fuel pump was discovered to be the problem on the e90's, dealerships were replacing the injectors periodically and they were about a $1,000 a piece.

[Dj Buttchug]

peep this. The pictures wont work and I cant take the time to host them right now. I will after work.


AUTOZINE TECHNICAL SCHOOL



High Compression Engine - Mazda Skyactiv-G

Higher compression ratio brings higher combustion efficiency hence power. That's why automotive engineers want to raise compression as high as possible. However, a compression too high will lead to early explosion of fuel-air mixture, or what we call "knocking". Knocking is bad to engines, not only because it causes NVH but also it reduces output. When I started reading about cars, most engines in the world ran at lower than 10:1 compression. As engine management and valve-timing technology improves, nowadays the figure can be higher than 11:1. Direct injection engine may even lift that figure to 12:1 or so thanks to its cooling effect, but anything higher than that remains a dream. However, Mazda made a breakthrough with its Skyactiv-G engine in 2010. It works at an incredible 14:1 compression !

How can Mazda avoid knocking? A crucial factor causing knocking is the high temperature of combustion chambers. Temperature in the chamber rises during compression stroke. It peaks when the piston reaches the top dead center (TDC, i.e. the highest position). At this point, knocking is most likely to occur. Obviously, if we want to reduce the risk of knocking, we had better to lower the combustion chamber temperature.

Then why is the combustion chamber so hot ? One of the reasons is the existence of residual exhaust gas, i.e. the exhaust gas that flows back into the combustion chamber during the intake stroke just before the exhaust valves close. No one can completely get rid of residual exhaust gas, because for high breathing efficiency engines always need to run with a certain level of valve overlapping (overlapping between the opening period of intake and exhaust valves). Suppose exhaust gas is 750ºC and the fresh intake air is 25ºC, and their mixture ratio is 1 to 10, you can see the residual exhaust gas can raise the combustion chamber temperature a lot. The more the amount of residual exhaust gas, the higher the combustion chamber temperature is. In other words, if we want to reduce temperature, we can reduce the amount of residual exhaust gas in the combustion chamber.

This graph shows that a 14:1 compression engine always has higher comnbustion chamber temperature than a 10:1 engine on a given residual exhaust gas level. However, if the amount of residual exhaust gas is reduced to 4 percent, combustion chamber temperature will be about the same as a 10:1 engine running with 8 percent of residual exhaust gas.

Now the question is: how to lower the percentage of residual exhaust gas?


Surprisingly, Mazda uses a very conventional approach to do that: a long, 4-to-2-to-1 exhaust manifold. On a typical inline-4 cylinder engine with short, 4-to-1 exhaust manifolds (the first picture), once the exhaust valve of Cylinder 3 opens, its exhaust pressure waves (grey area) flows through the short manifolds to the exhaust valve of Cylinder 1, which is at the end of its exhaust phase. This pumps some exhaust gas back into Cylinder 1 and becomes residual exhaust gas. When the engine is running at low speed (2000 rpm in the first picture), the exhaust pressure wave arrives Cylinder 1 early enough to cause high percentage of residual exhaust gas. As engine rev rises, the opening and closing of valves speeds up as well, thus the exhaust pressure waves of Cylinder 3 reaches Cylinder 1 at later stage, causing lower percentage of residual exhaust gas. In short, from low to mid-range engine speed the level of residual exhaust gas is pretty high for this engine configuration.

In the case of Skyactiv-G's 4-2-1 exhaust manifolds (the second picture above), exhaust pressure waves from Cylinder 3 has to travel a long way to reach Cylinder 1, by the time Cylinder 1 has already, or nearly completed its exhaust phase. Therefore the level of residual exhaust gas is much lower than the previous case, especially for low to mid-range rpm. As a result, the Skyactiv-G engine attains lower temperature in its combustion chambers and allows a higher compression ratio to be used.

Well, if the principle is so simple, why not others discovered already? It's not that simple, of course. One critical drawback of the long 4-2-1 exhaust manifold is that it takes relatively long time to heat up the NOx catalyst during cold start. In fact, this is exactly the reason why most modern production engines have abandoned this exhaust configuration - with the exception of high-performance engines which may use thin-wall fabricated stainless steel exhaust manifolds to compensate for its extra length. On cost-conscious mass production engines, cheap cast-iron exhaust manifolds are still the norm. Its extra mass and surface area absorb a great deal of heat and delay the proper functioning of catalyst. This causes difficulty to comply with emission regulations.

Mazda overcomes the cold-start problem by retarding ignition. This leads to a higher exhaust gas temperature to compensate for the long manifolds. The late ignition may result in unstable combustion. This is dealt with a specially shaped piston which concentrates the stratified air-fuel mixture around the spark plug. Other supporting features like high-pressure direct injection and six-hole injectors also contribute to the optimized combustion.

[Vspec]

I drove the 5dr and was sold on the spot, butter smooth shifting 6spd, stellar fuel economy: 2750rpm at 120kph (75mph). Low roofline kills cargo space however. Nicely equipped too.

But we all know FWD< RWD!

[pandaboo911]

I thought that skyactiv was a bunch of marketing garbage. Surprised to see some actual engineering behind it, even though scavenging exhaust gases in headers is nothing new..

[Zzyzx85]

as long as mazda takes care of the carbon build up issue that seemed to plague to the MS3, I think the new skyactiv engine should be a winner.

my friend's wife is getting about 35mpg on their Mazda 3 skyactiv that's still breaking in. she drives slow (55-60 mph in the slow lane) but not bad considering she's driving in moderate traffic.

[MattBimmer]

Again lots of thanks, especially to you Mr? Or Mrs? ButtChug lol. I feel much better knowing that they are at least trying to fix the carbon problem, and I think a catch can won't be hard if needed.