[Diy_efi] Intro, and q's about fuel economy

[Perry Harrington]

> 
> Ya, there's no emissions stuff.  We run with a straight pipe and no
> catalytic conv.

You ought to consider a couple of things.  OEMs get better mileage by
recycling the exhaust gases (EGR).  The EGR allows you to reduce your
fuel load and run leaner.
> 
> An all analog system is certainly intriguing.  I think it'll be easier to
> develop the system with a programable board, but if we get everything
> working before the competition, I'll start looking into a simpler system.

Using an analog system is of little benefit if you've achieved EFI.  The whole
point of EFI is that you can do things analog can't.  Just because most
aftermarket or homebrew systems don't implement trick algorithms or strategies
doesn't mean you have to forget them.

If you don't already have an EFI expert (or Internal Combustion Engine expert)
on the team, then you would do well to find one.

> Oo, that's a good suggestion for research.  I think it's applicable... in
> the competition, we open the throttle, get the car to x mph, then kill the
> engine and coast to y mpg, then start the engine and run up to x ... etc.

The Coast and Burn technique is oft used for ICE competitions.  If I were to
want to use as little fuel as possible, I'd work on these things:

A powerful electric motor for the major acceleration.  This is almost free if
you have a large battery and an alternator that can recharge the energy released
in the burn.  You could run an engine at a static load once accelerated up by
the electric motor to recharge the battery, or you could put a very high gear
ratio (numerically low) between the motor and the drive.  This would highly benefit
torquey motors and allow a long sustained accelleration run.  You would gain RPM
very slowly but it would give you time to recharge the battery as well as reach
speed.

I don't know if your competition has specific fuels mandated, but if you search
the net you'll find a list of fuels and their specific energy densities.  Usually
these are rated in BTU per a unit of measurement (pound, liter, gallon, etc).

You may want to consider a fuel with a higher energy density, assuming that you
have an engine that can efficiently release that energy.

If you are using conventional piston engines there are plenty of tricks to reduce
the friction in them.  Ironically most of them are counter to new-engine building.

To reduce friction you setup your clearances fairly wide (main and rod bearing
clearances, piston wall clearances, low viscosity oils).  The one place where you
may be tempted to play with clearances is piston rings, but I'd recommend against
it, as this will reduce your efficiency.  I'd actually recommend gapless rings
to help retain as much combustion pressure as possible.

A little piece of advice on engines:  the longer the stroke, generally the more
torque they make for a specific displacement.

Large bore engines are good for high airflow requirements and horsepower.  I'd
recommend against achieving high horsepower and go for high torque.  Torque is
directly related to cylinder pressures and engine efficiency.  HP is a measure
of airflow essentially.

An engine that makes lots of torque at very low RPMs is preferrable as you will
use less fuel at lower RPMs than higher RPMs, since your airflow is less (in NA
engines).

Also, you didn't specify if the competition disallows enhancers or modifiers to
the engine's intake of fuel/air.

Modifying the fuel or air by use of Nitrous Oxide or NitroMethane can significantly
improve your efficiency.  Nitrous Oxide is 2 parts nitrogen and 1 part Oxygen, when
it's burned 2 molecules of N2O combine to make 2 N2 and 1 O2, increasing the pressure
in the cylinder and introducing a 50% oxygen volume (air is 21% oxygen).  N2O makes
for very high cylinder pressures and is a great way to improve torque output and
reduce total fuel requirements.

Nitromethane has similar advantages as N20, but others too.  Nitromethane is 50%
oxygen, so it has similar performance characteristics to N20.  The most common
fuel combined with Nitromethane is methanol.  This has the effect of lower the
octane of the methanol and also reducing combustion chamber temperatures because
of the primary ingredient is methanol.

I don't know how Nitromethane reacts with other fuels or how it reacts with N2O,
but both of these modifiers require specially prepared engines to fully take
advantage of the performance of these modifiers.

N2O generally requires less preparation than Nitromethane, but an engine that is
designed to run with Nitromethane will most definitely work well with N2O.

Another modifier used on Diesel engines is to inject propane into the intake air
stream.  This has the effect of greatly improving the combustion efficiency and
torque.  There are many commercial kits for light duty trucks.

> 
> What're BSFC and WOT?

Brake Specific Fuel Consumption and Wide Open Throttle.

BSFC is the ratio of fuel per HP of engine output.  Values range from .3 to .9 in
most applications you will encounter.

Values and applications examples:

lb per HP	Application
.3		Aviation ICE (static loads for long periods)
.45-.5		NA ICE (Naturally aspirated)
.6		Forced induction (turbo, supercharger) ICE
.7-.8		Wankel (rotary) engines IIRC
.9		2-stroke ICE engines

I don't have any BSFC numbers for Diesel or Turbine engines.  Turbines are higher
and Diesels are lower, both inherent to design and fuel.  Turbines run on a Kerosene
like fuel, thinner then Diesel but similar properties.

> 
> -j
> 

--Perry

-- 
Perry Harrington			Data Acquisition & Instrumentation, Inc	
perry at dainst dot com					 http://www.dainst.com/

Those who would give up essential liberty to purchase a little temporary safety
deserve neither liberty or safety. Nor, are they likely to end up with either.
                             -- Benjamin Franklin

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