[Diy_efi] RE: Diy_efi digest, Vol 1 #413 - 12 msgs

[Eric Deslauriers]

Shannen,

Wow! OK, I owe you a beer. That was EXCELLENT! :)

Eric D

> -----Original Message-----
> From: diy_efi-admin at diy-efi.org [mailto:diy_efi-admin at diy-efi.org]On
> Behalf Of Shannen Durphey
> Sent: Thursday, December 19, 2002 11:52 AM
> To: diy_efi at diy-efi.org
> Subject: Re: [Diy_efi] RE: Diy_efi digest, Vol 1 #413 - 12 msgs
> 
> 
> Didn't you once say you specialize in asking questions?  This is a great
> one.  It's funny how we have an instinctive grasp of engine load, and
> yet the definition can get very slippery when we try to put our fingers
> on it.  
> 
> I should probably look around for an official definiton for engine
> load.  I would say, with some thought, that load is resistance to
> acceleration.  In this way, 100% load describes a situation in which the
> engine is unable to accelerate, and zero load (which is physically
> impossible) describes a situation where the engine can change speed
> instantly.  Also, an engine under zero load is unable to produce either
> torque or power, as those values are both defined in part by
> acceleration.   This definition also allows load to be independent of
> power and torque.  
> 
> In regards to the dyno, with load defined as resistance to acceleration,
> we can apply an inertial load by using the tendency of mass to resist
> acceleration, or we can apply a resistive load, which is a force, to
> maintain a desired acceleration.  The resistive load can be measured
> directly, as in a friction brake dyno, where power and torque values can
> be worked out from the known force.  In the case of the electric brake
> dyno, we can measure power required to maintain a desired acceleration
> directly and calculate torque.  We generally don't take into account any
> friction made at the contact patch between tires and rollers.  Instead
> we calculate or measure the power required to produce acceleration at
> the roller and assign that to the power produced by the wheels.  When
> the dyno is producing an "8hp load" it's really creating a force equal
> to the force applied by the wheels, but opposite in direction, and so is
> creating a negative acceleration.  Load on the street would be a
> combination of the inertia of the drivetrain, force of gravity, wind
> resistance, and friction.
> 
> The true measure of load is change in rpm.  We are lucky to have a
> natural ability to modulate engine power to meet load without even
> thinking aboout it.  We can sense a change in acceleration and apply
> more or less force to the road to meet that change in acceleration.  How
> we apply more or less force is by modulating the throttle.  So we
> associate load with throttle position.  If we grew up driving 2500 lb
> vans with 10 horsepower engines, we might not have that association. 
> The throttle would in that case be almost always fully on or fully off,
> and we might only tend to think of situations where we need to apply
> negative acceleration to slow down.  We also associate load with MAP, as
> we know there's a fundamental air flow relationship between throttle
> restriction and the pressure across the restriction.  When MAP is high,
> it infers less restriction, which must mean we're attempting to overcome
> a large load.
> 
> The issue between "large" and "small" loads shows up over time.  We live
> in a world of finite distances.  To cross these distances, we tend to
> assign an expected time.  Even without a watch, we have a sense of speed
> that we use, which measures the time it takes to cross a percieved
> distance.  With our grasp of speed and time, we modulate power to create
> the acceleration we need to cross the percieved distance in the expected
> time. : )
> 
> So where does this lead?  If we had a perception that was directly
> related to the energy released by consuming fuel, in addition to the
> velocity and acceleration that results from burning fuel in the engine,
> then we might know without thinking that not all energy released in
> combustion creates a force on the crank.  A fair amount of that energy
> heats parts.  
> 
> Adam mentioned earlier that there's a fixed rate of heat transfer
> through metals.  When an engine has been operating for a long enough
> time, the temp of the cooling system + the temperatures of the heads and
> pistons + the energy vented out the exhaust + the power output at the
> crank has reached an equilibrium with the energy  produced by
> combustion.  If we now desire to cover a distance in less time, produce
> an acceleration, we consume additional fuel, convert additional energy,
> to produce that acceleration.  If the duration of this event is short,
> we don't make large changes to the equilibrium of the of the cooling
> system/heads/wasted energy/crankshaft power system.  There isn't enough
> time to significantly heat the cooling system and heads, and what isn't
> used at the crank ends up in the exhaust.  As the time through which we
> accelerate increases, so does the time which excess heat energy can heat
> the cylinder heads and cooling system.  And the slower the acceleration,
> the longer it takes for the exhaust and intake cycles, the less heat
> energy is removed from the cylinder through these means.
> 
> Power production drops as density of the air entering the combustion
> chamber drops.  Power maximized by adjusting the start of the burn to a
> specific time will drop when the rate of burn changes.  If we go from
> combustion to detonation, power production drops off rapidly, engine
> damage is looming.  Increasing the heat level of the system will
> generally decrease intake air density, decrease burn times, and
> aggravate detonation.  It's important to fight these gremlins.
> 
> Decreasing air density suggests decreases in fuel.  Increasing burn rate
> requires delaying the start of the burn.  But the real killer is
> detonation.  No matter how carefully we adjust fuel and spark, if we end
> up with detonation we lose power and eventually parts.  To prevent
> detonation, we have some options:  1) reduce the time we spend
> converting energy, that is, lighten the vehicle or let off the throttle.
> 2) alter the relationship between the engine's acceleration and our own:
> change gearing, alter the load on the engine.  3) reduce the energy we
> produce:  add an inert gas to the combustion process, for example  4)
> try to keep the heat energy away from the cylinder heads.
> 
> Most tuning at the dyno centers around option 4.  We already work to
> keep heat energy away from the heads during acceleration by providing a
> mixture rich in fuel.  A/F mixtures richer than roughly 14.7:1 don't
> react any more fuel than mixtures at 14.7:1.  But they absorb heat from
> the combustion process and affect the burn rate.  So if we have a
> situation where we're getting additional heating of the chamber, we can
> add more fuel to absorb the heat, displace oxygen, and decrease burn
> rate.  The trick is to add just enough fuel to prevent detonation and to
> not add so much that power drops unacceptably. 
> 
> Gear ratio compensations _should_ be time compensations.  I've never
> used an ecu with a gear ratio compensation function, so they might be
> just that.  But the time compensation should be (imo) to first reduce
> spark then add additional cooling media (fuel, water, alky, whatever) as
> the amount of time under load increases.  For control systems which do
> not have time compensations, the tuner must estimate if and when
> cylinder heating will cause detonation.  And the tuner must add the
> necessary amount of fuel to prevent detonation before it happens.  Which
> means that if the tuner is wrong, or if the conditions under which he's
> made his estimation are substantially different from the operating
> conditions of the vehicle, his tune is less than the best.
> 
> <whew>
> 
> <snip>

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