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'[OT] Re: Water injection with one mention of PIC'
| A bit off the PIC subject, but interesting nonetheless.
On Mon, 15 Jun 1998 17:16:01 +1000 Clyde Smith-Stubbs <HTSOFT.COM> clyde
>The The use of water injection with petrol engines seems
>to mainly be useful with turbo- or super-charging, and allows higher
>ratios to be used without causing detonation. So it's a means of
>more power, but it wouldn't help a normally-aspirated engine much, I
>and I can't see it would do anything for fuel economy.
The problem of early ignition, thus requiring all sorts of weird
chemicals in the fuel and limiting the compression ratio seems to me to
be due to the fuel being in the cylinder during the compression stroke.
I really like the idea of a diesel engine. Put the fuel in there when
you're ready to burn it! The burn can be controlled to be throughout the
power stroke by metering the fule into the cylinder during the stroke.
Takes a strong injector, but seems like it could be done.
On water injection, I seem to recall that a gasoline engine is
not really a "hot air" engine where the power is due to the expansion of
a gas, but is more of a steam engine where the power is due to the
vaporization of excess fuel. This can be demonstrated by running a lean
mixture, which causes the engine to run hot. The heat that had been used
for vaporization of the excess fuel is now heating the block instead of
vaporizing the excess fuel, since there is less (or none). It seems that
water can be substituted for this excess fuel to turn this heat into
useful energy. I'm no expert at internal combustion engines... this is
just my wild imagination! It makes sense to me, though.
>The fuel economy of a car depends
>on such factors as how fast you drive (air resistance increases as a
>the speed, I think it's the square theoretically, in practice it can
>higher), how much energy you lose through braking, etc.
I think you can take the energy in the fuel, multiply it by the
engine efficiency, then start subtracting losses (tire rolling
resistance, air resistance, transmission losses, etc.). Without these
losses, it would take no power to maintain a certain speed. You'd just
take a certain amount of energy, dump it into the mass of the car to get
the equivalent kinetic energy of (1/2) m v^2. When you stop the car, the
energy gets dumped into the brakes and turned into heat.
As I recall (from reading about force based wind speed
indicators), the force on an object is proportional the the square of the
wind speed. We then multiply this force by distance traveled to get the
energy used to push the object thru the air. We can get the power
required by dividing that energy by the amount of time the energy was
used over (power being the rate of energy consumption or generation). If
we double the speed, we double the force required to push the object thru
the air. However, we ALSO double the distance the object travels in a
unit time. Therefore, the energy required in a unit time (the power) is
multiplied by 8 (4 for the force increase, 2 for the distance the force
was applied). So, power requirements increase as the cube of speed;
energy requirements to go from one point to another increase as the
square of the speed (assuming the major loss is wind resistance). It
would be interesting to develop a model for a typical internal combustion
car. It appears that thus far "efficiency" gains have been made by
decreasing losses instead of by making major increases in the efficiency
of the engine.
An employer is working on an electric car. I've also got a
former employer who has built a couple. There's also a solar power car
club at the local university. None of these people have done an "energy
budget" on their vehicles. I think it would be interesting to write a
model for power consumption of the car based on wind resistance (doing
tests under various speeds and wind conditions), tire rolling resistance
(I'd hope data would be available from the tire manufacturers), and tests
on transmission losses (perhaps seeing how much power it takes to spin
the wheels various speeds with the car up on blocks). I'd then like to
"meter everything" (using a PIC, of course), and dump the data into a
notebook computer. The "meter everything" would pretty much be the power
into the motor, measured wind speed, position (using GPS), and perhaps
"altitude" if GPS doesn't appear accurate enough (it takes energy to go
up a hill, but you get it back going down the hill where it is usually
used to recover losses: you don't need as much motor power to get the
same speed going down the hill as you do on level ground or going up the
Once another one of these cars is done, I'll start metering
everything and see if I can come up with a good model.
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|On Mon, Jun 15, 1998 at 02:39:48PM -0400, Harold Hallikainen wrote:
> I really like the idea of a diesel engine. Put the fuel in there when
> you're ready to burn it! The burn can be controlled to be throughout the
> power stroke by metering the fule into the cylinder during the stroke.
No need - diesels essentially operate with controlled detonation. The problem wi
a petrol engine is that the fuel will ignite before it's wanted if the compressi
ratio is too high, but a diesel controls this by not injecting the fuel until
it's ready to be ignited. That's one reason they're noisy - they're detonating
all the time.
> On water injection, I seem to recall that a gasoline engine is
> not really a "hot air" engine where the power is due to the expansion of
> a gas, but is more of a steam engine where the power is due to the
> vaporization of excess fuel. This can be demonstrated by running a lean
Not true. Some engines (e.g. aero engines) need a rich mixture at max power
to provide extra cooling, but at cruise (60-75% power) can be leaned to peak
EGT (exhaust gas temperature) or even beyond, where the EGT falls as the
mixture is leaned (power also falls, but economy can improve). Modern auto
engines run much leaner than those of 30 years ago, one reason for the
much improved fuel economy and lower emissions.
I've seen nothing to suggest that water injection recovers more energy
from the burning fuel/air mixture, it just allows you to stuff more in
there in the first place (and injecting (m)ethanol with it provides extra
fuel as well).
> car. It appears that thus far "efficiency" gains have been made by
> decreasing losses instead of by making major increases in the efficiency
> of the engine.
Not quite; modern engines are more efficient (due to leaner mixtures and more
accurate fuel metering) than older ones, but the gains have been evolutionary
rather than revolutionary. I'm not aware of any revolutionary technologies in
the wings, but incremental improvements will probably continue for a while.
Clyde Smith-Stubbs | HI-TECH Software
Email: htsoft.com | Phone Fax clyde
WWW: http://www.htsoft.com/ | USA: (408) 490 2885 (408) 490 2885
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HI-TECH C: compiling the real world.
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