Originally Posted By: Silk
Originally Posted By: Virtus_Probi
You see lots of diesels with monster peak torques but more modest peak HPs because their torque peaks early in the RPM band...I'm sure there's a good explanation for this, but I don't know what it is.
Diesel is a long burn, it will continue to push that piston down long after a petrol burn has gone out, that doesn't work so well at high speeds.
Here's a pic of the pressure an volume curves of a couple of theoretical engines (otto and diesel) - note, of the SAME COMPRESSION RATIO.
Ideal otto, the fuel is compressed to TDC (minimum volume), then all burns instantly, which is the vertical spike on the curve 1, 2, 4, 7.
Ideal diesel, it gets to the same point, then the fuel is introduced slowly over time, and forms the curve 1, 2, 6, 7...and as this takes heaps of time, as silk says, limits speed.
The area within the curve is the energy available per cylinder power stroke to make power.
Real life diesels, the Compression ratio (point 2) is much higher, so the diesel is not as much hindered as the area within the curve suggests.
Real life ENGINES, the combustion process is neither instant (theoretical otto), nor flat (theoretical diesel), but takes closer to the 1, 2, 3, 5, 7 approach egardless of design, and as Mazda are demonstrating, otto C.R.s are going up, and diesel going down, and with 7+ separate injection events per cylinder for modern diesels, they are nearly the same.
My Nissan ZD30 (EGR delete, as it couldn't do it stock) will pull away at idle, and accept a 1-2 shift on the flat with no throttle movement at any stage...wet grass, just let the clutch out quickly in 1st at idle, then chnge 2nd, and it will never flinch, nor gain traction.