אני לא אומר שאתה לא צודק. אני אומר שאתה לא לגמרי צודק. כמו שאמרתי, במנוע בנזין (יכול להיות גם בדיזל, תלוי בתכנון המנוע) זה שילוב של דחיסה ללא פיצוץ בפעימת העבודה פלוס הואקום שנוצר.
הנה חישוב שמדגים את זה:
This is the "conventional wisdom" that you'll read pretty much everywhere on the Internet. Unfortunately it's a little incorrect. It totally neglects the heat generated in the compression cycle. Any heat increase is a loss in pressure (P*V = nRT). When we decrease volume (compress), both the Pressure and Temperature increase (it's an inefficient process, if it were totally efficient, there would be no temperature increase and we would get full pressure and it could then have the full "spring" effect. I've some back of the envelope calculations to try and put it into perspective. My example is for a 2L diesel motor, with say a 19:1 compression ratio. That should get you somewhat over 600 degree increase in cylinder temperature.
Temperature increase 600 Degrees
Volume/cylinder 500 mL
Specific density of air (kg/m3) 1.184
Volume 0.0005 m3
Mass of air 0.000592 kg
0.592 g
.
Specific heat capacity of air ( kJ/(kg.K) ) 1.047
.
Energy consumed (per revolution) 0.3718944 kJ
Revolutions 2000 /min
. 120000 /hr
.
Total energy 44627.328 kJ/hr
.
Convert kJ/hr to kW divide by 3600
.
. Power 12.39648 kWSo in this example we would have the equivalent of a 12 kW motor acting as a brake. At 3000 RPM it would be 18 kW. NB. These calculations ignore the boost pressure from the turbo (say 2 atmospheres). That would effectively double the braking power. This also doesn't include extra "braking" power from the DPF. There will be extra losses from having to force the exhaust gas through it. Obviously not in the same league as a Jake brake, but nothing to be sneezed at. [EDIT]
Some notes: Technicaly the power then needs to be converted to Torque at the wheels to figure out the exact braking force. But that is only a funciton of the RPM at the wheels so raw power is somewhat relevant. The 12 / 18 kW (or more) brake motor is the equivalent a motor of that power "at full power" (and thus full torque as per the gear ratios, as mentioned above). You will actually be getting all of that braking power at the wheels. It's a bit like a heat pump. They are fully efficient, unlike an air conditioner. Just imagine your 100 / 120 kW diesel car with your foot flat to the floor. That's just to put it into perspective. There are also a bunch of frictional forces that assist braking on top of the engine braking.
[/EDIT]
לגבי מנועים ישנים אתה שוכח כמה גורמים כמו גיל, סוג תיבה, מערכת יניקת האוויר, חיכוכים במערכות השונות אווירודימיות של הרכב ועוד.