>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] 
>On Behalf Of Rich
>Sent: 28 June 2007 06:35
>To: Microcontroller discussion list - Public.
>Subject: Re: [EE] Simple fuel-saver, so they say
>
>
> The expanding forces against the side walls of the cylinder do not 
>contribute to the force on the piston head and so contribute to the 
>inefficiency.

I'm not sure this can be true.  The very fact that the cylinder and cylinder head are present is what creates the pressure in the first place.  Exterting pressure on a static body performs no work.  What does reduce efficiency is the cylinder, head and piston absorbing heat from the burning fuel/air mixture, which is why ceramic coatings have been developed to try to reduce this (for piston and head at least).


>In diesel engines the compression ratio is much 
>higher and it 
>is so oxygen rich that a spark is not required to initiate the 
>burn.

The reason it does not need a spark is because the heat created during compression is used instead.  This heat would also readily ignite a stoichiometric gasoline/air mixture, but the resulting burn would not be controlled, e.g. detonation.


> The reason diesel can 
>tolerate 
>higher oxygen ratios is because there is less BTU per unit 
>volume than for 
>gasoline, of whatever octane.

Diesel actually has a higher energy content than gasoline.  Also note that octane rating is not an indicator of energy content of gasoline! In fact very often the high octane fuels will have slightly lower energy density, because the additives required to increase the octane rating displace some fuel (e.g. what used to be tetra-ethyl lead many years ago).  High octane simply means that the fuel has improved resistance to detonation, so a higher compression ratio can be used to extract more energy from the fuel.

The reason that very weak (i.e. excess oxygen) mixtures can be ignited in a diesel is because they do not rely on a small spark located at one point in the combustion chamber, instead the fuel is injected into the compressed and heated air and starts burning immediately.  In a gasoline engine, the mixture has to have enough fuel for the spark to initiate combustion.  This is why stratified charge engines have been developed, this creates a small fuel rich mixture around the spark plug, surrounded by a larger volume of weak mixture.  Once the rich mixture is ignited, the flame front will happily propagate through the weaker regions. The GDI engines that e.g. Mitsubishi have developed rely on this.

Note that the actual mixture ratio in a diesel varies considerably over the engines operating range because it not throttled, i.e. the volumetric efficiency is more or less constant, only the amount of fuel injected varies.

Regards

Mike

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