The assumption I have made here is that combustion happens with 10 deg. of TDC,
which I am unsure about. It may not actually be complete in that time.
I tried to track this down (with little actual success) and the few
vague assertions I could find made it seem like combustion actually
takes a lot longer than that. Sites I found made it appear that it
completes at least 10 degrees after TDC, which means it's taking more
like 20 degrees than 10.
OTOH that's in the ideal case, and if the chamber has hotspots or it's
running on gas that's too low test or any of a number of other things,
combustion may run rather faster. And the original assertion, IIRC, had
to do with the /peak/ combustion rate in the engine (as an example of
chemical energy production rates), so if it hits your numbers only when
overheating while accelerating up a mountain pulling a trailer with a
tank full of Gulftane, that's good enough. :-)
On 06/17/2016 07:54 PM, [email protected] wrote:
In reply to Jed Rothwell's message of Fri, 17 Jun 2016 18:00:10 -0400:
Hi,
[snip]
<[email protected]> wrote:
Rubbish. The ignition of the fuel in a normal car engine cylinder results
in a
power production on the order of half a megawatt for a couple of
milliseconds,
and that's just chemical energy (with maybe few hydrinos thrown in ;).
Really? Hmmm . . . How do you figure that?
At 60 mph a typical old car consumes ~20 mpg. 1 gallon is 3.8 L, so that's
0.19 L per mile (or per minute). 1 L of gasoline produces 34.2 MJ. Divide
by 20 to get 1.71 MJ per minute.
Using your figures of 34.2 MJ/L and 0.19 L/minute I get 6.5 MJ/min, however
that's for 6 cylinders, so for 1 cylinder that would be about
1 MJ/min = 18 kJ/sec.
A single piston stroke produces only a
small fraction of that. I don't know how long it takes the gasoline to
burn, but I doubt such a small amount would reach the half-megawatt level.
Here is a paper on the burn rate of gasoline, which you have to pay for:
http://papers.sae.org/2008-01-0469/
At 60 mph, I think engines run at about 2500 rpm. A single piston stroke
with a 6-cylinder engine running at 2500 rpm would consume . . . ummm . .
.1.71 MJ / 15,000 = 114 joules. Right? That takes only 0.0002 s to burn?
- Jed
A 4 stroke engine has one power stroke/cylinder every 2 cycles, so at 2500 rpm,
that's 1250 power strokes/cylinder/min = 21 power strokes/cylinder/sec. So each
power stroke/cylinder produces 18 kJ/21 or about 860 J.
At 2500 rpm, there is a revolution (360 deg) every 24 ms.
If combustion happens within 10 deg. of TDC, then combustion time is
24 ms*(10/360) = 0.67 of a ms.
860 J / 0.67 ms is about 1.3 MW.
The assumption I have made here is that combustion happens with 10 deg. of TDC,
which I am unsure about. It may not actually be complete in that time.
The figures I used initially were a little different to yours, hence my original
calculation was also somewhat different.
Regards,
Robin van Spaandonk
http://rvanspaa.freehostia.com/project.html
