A few moments from a job interview… 

“So SKYACTIV G is all about the increased compression ratio!”
“Yes… increased compression ratio is the main thing. 4-2-1 exhaust, cavity pistons and other things you mentioned are to enable the high compression ratio”
“14:1! Are you sure?”
“That is what they claim.”
“What are they trying to achieve with this increased compression ratio?”
“Increased thermal efficiency! 9% increase if compression ratio is raised from 10:1 to 15:1.They got 15% increase in fuel efficiency and torque. The 2012 model Mazda3 is in the elite 40mpg club now”
“If it is that easy then why others are not doing this? Increasing compression ratio is not a big deal!”
“The problem is with knocking. High compression ratio increases the probability of knocking”
“Then?”
“Knocking has to be controlled”
“How they are doing it?”
“By reducing the amount of residual gas”
“Residual gas!?”
“It is the amount of hot exhaust gas that remains inside the combustion chamber”
“How this is related to knock?”
“The amount of residual gas can have an effect on the compression temperature. If the residual gas is reduced, the compression temperature will be reduced. According to Mazda, if the residual gas is halved, from 8% to 4%, the temperature at compression TDC will remain the same even when the compression ratio is increased from 11:1 to 15:1”
“Interesting… could you tell us how they are controlling this residual gas?”
“It is the 4-2-1 exhaust system… another innovation… When exhaust manifold is short, the exhaust gas emerging from one cylinder during the beginning of exhaust stroke can enter another cylinder which is at the beginning of intake stroke and whose exhaust valve has not closed yet, resulting in increased residual gas. By increasing the length of the manifold…”
“Sorry… I am not getting you…”
“I mean… let us think of a 4 cylinder in line engine with firing order 1-3-4-2.The exhaust valve timing is such that the exhaust valve closes 5 deg ATDC. Clear?”
“Yes”
“Consider the beginning of intake stroke in cylinder 1. What will be the position of exhaust valve? Open or close?”
“Open. About to close”
“Now what will be the situation in cylinder 3?
“Exhaust? Beginning of exhaust stroke??”
“Correct… it will be exhaust stroke beginning. The exhaust valve in cylinder 1 is not closed, it is the beginning of suction stroke there, and the cylinder 3 is pumping out hot exhaust gases into the exhaust manifold. Is it clear?”
“Clear! You mean to say that some of the exhaust coming from cylinder 3 will enter cylinder 1?”
“Exactly… But we can control this by using longer exhaust manifold”
“You mean longer pipes?”
“Yes… long pipes… 600 mm minimum… They call it 4-2-1 exhaust system”
“Four 600 mm pipes! It will be taking up a lot of space! Won’t it?”
“True… they are adopting a loop shape and utilizing the space efficiently. But there is another major issue…”
“What is that?”
“Longer pipes cool the exhaust gas before it reaches the catalytic converter. So the catalyst’s activation will be delayed”
“True! Interesting!! Tell us how they tackled this”
“They just increased the exhaust gas temperature… by delaying the ignition timing… but it leads to another problem… too much retardation makes combustion unstable. But they overcame this also… by charge stratification with cavity pistons and optimized fuel injection”
“Hope there are no more problems… Bye the way, you had any road tests?”
“Yes! With the CX5”
“Is it still fun to drive?”
“Hahaha… no compromise on that! It is still fun to drive…like its siblings… and the fun does not end when you are driving to gas station.” 

A couple of questions on the Atkinson and Miller cycle engines and it was over.

Sorry… I can understand your curiosity… but I can’t reveal which side of the interview table I had been.

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