Attention: TDR Forum Junkies 
rbattelle
TDR MEMBER
I recently read a good paper in the JSAE (Japanese SAE) entitled "Study on Pilot Injection of DI Diesel Engine Using Common Rail Injection System" (Tanaka, Ando, and Ishizaka). It was quite interesting, and I thought I'd mention some things from it here. For copyright purposes I have not directly quoted anything in the paper.
Of particular interest is what I would call the pilot injection paradox. On the one hand, the use of pilot injection can significantly reduce combustion noise and improve emissions performance. On the other hand, the pilot injection consumes some available combustion air, making less available to the main combustion. If you've got too much pilot, main combustion won't be active enough. Too little pilot, and you lose the emissions and noise benefits. The question is: what's the right combination of pilot timing and quantity? This paper attempts to answer it.
The experimentors tested three main conditions: low quantity, retarded timing; low quantity, advanced timing; and high quantity, retarded timing.
To make a long story short, they found that a small quantity and advanced timing provides a reasonable balance between emissions/noise performance, and overall performance. However, all things in moderation: too little pilot and too advanced timing tends to increase delay angle, which increases the tendency to detonate and increases combustion noise. It appears from their findings that a pilot advance angle of around -30 ATDC and small quantity (for their engine, a 4-cylinder Toyota, around 0. 000183 in^3, or 0. 0000061 in^3 per cubic inch of combustion chamber volume) provide the best solution.
I would post some of the pictures/graphs these guys had, but I think that would be illegal due to copyright issues. Anyway it was an interesting read and I thought I would just mention some of the findings for others to see. Enjoy!
Of particular interest is what I would call the pilot injection paradox. On the one hand, the use of pilot injection can significantly reduce combustion noise and improve emissions performance. On the other hand, the pilot injection consumes some available combustion air, making less available to the main combustion. If you've got too much pilot, main combustion won't be active enough. Too little pilot, and you lose the emissions and noise benefits. The question is: what's the right combination of pilot timing and quantity? This paper attempts to answer it.
The experimentors tested three main conditions: low quantity, retarded timing; low quantity, advanced timing; and high quantity, retarded timing.
To make a long story short, they found that a small quantity and advanced timing provides a reasonable balance between emissions/noise performance, and overall performance. However, all things in moderation: too little pilot and too advanced timing tends to increase delay angle, which increases the tendency to detonate and increases combustion noise. It appears from their findings that a pilot advance angle of around -30 ATDC and small quantity (for their engine, a 4-cylinder Toyota, around 0. 000183 in^3, or 0. 0000061 in^3 per cubic inch of combustion chamber volume) provide the best solution.
I would post some of the pictures/graphs these guys had, but I think that would be illegal due to copyright issues. Anyway it was an interesting read and I thought I would just mention some of the findings for others to see. Enjoy!
