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Posted by harry on August 22, 2009, 6:35 pm
 



A modern petrol engine and a modern diesel aren't very different as
regards efficiency. It was the carburretor that f****d things up in
the past

Posted by Bruce Richmond on August 22, 2009, 11:02 pm
 



Under light load conditions a diesel has a great advantage in that it
does not have to draw air in past a nearly closed throttle plate.
That saves it from doing a lot of work.

Another reason for the diesel's higher efficiency is its compression
ratio.  I forget the formula for theoretical efficiency right now, but
CR was a major factor.  In the days of old it wasn't unusual to find
diesels with 22:1 CRs compared to gasoline engines with 7:1.  But
those high CRs required heavy parts to withstand the forces and
tempertures involved.  Those heavy parts made for a heavier vehicle
and limited the working speed of the engine, restricting its power
output for a given size.  Diesels also tended to have small bores and
long strokes so that they could achive those high CRs, which also
tended to limit their rpm capability.

Over the years CRs in diesels has dropped allowing them to be made
lighter and with bore to stroke ratios more like gasoline engines.
That has allowed them to rev higher and produce more hp for a given
engine weight.  At the same time gasoline engines have gone to higher
CRs.  This was allowed in part by better fuel mixture control from
fuel injection.  Other major factors are better spark timing and
combustion chamber shapes that help avoid detonation.

So yes, the efficiency of the two as used in vehicles is closer than
it was.  But if we are talking a stationary diesel running a generator
there's no reason to worry about the power to weight ration and skimp
on strength.  Give it a high CR and let it chuff away at low rpm to
reduce internal losses.

Bruce

Posted by Frank on August 21, 2009, 12:18 pm
 

Curbie wrote:

Offhand, I can't see any fault with his reasoning.  If he were in the
far south, he could probably avoid blending with gas but in Kansas,
where it can get cold, viscosity or gelling would be a problem in cold
weather with pure seed oils.  He's also not paying fuel taxes.

I've seen biodiesel for sale at a gas station in PA for about the same
price as regular gas.

Posted by Curbie on August 21, 2009, 11:11 pm
 

Frank,


There seems to be two issues going on here:

1) raising the viscosity of the fuel. (cold weather)
2) raising the volatility to completely burn glycerin and avoid
combustion-chamber carbon-buildup (a problem common to diesels running
with SVO).

The thing that has me confused with this, is that I would think that
once enough gasoline has been added to SVO to achieve proper
combustion, it seems that adding any more for to adjust for cold
weather viscosity would be risky for combustion?

Seems like there may be some here, but I just can't get my mind around
it yet?

Curbie



Posted by Bruce Richmond on August 22, 2009, 2:10 am
 


I'm just thinking out loud here so I may be way off but,

Thick heavy oils have long carbon chains.  Gasoline has mostly shorter
carbon chains, is thinner and burns quicker.  Could be that the
gasoline is not only adjusting the viscosity but also the burn rate/
temperture.

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