From: ccw@slee07.srl.ford.com (Chris Warren)
Subject: Re: Fuel octane versus power
Organization: Ford Motor Company Scientific Research Labs, Dearborn, MI
Date: Wed, 8 Apr 1992 17:10:44 GMT

koziarz@halibut.nosc.mil (Walter A. Koziarz) writes:

	--stuff  skipped---

: simplest explanation --
: 
: higher octane fuel contains *less* energy per unit volume than does lower
: octane fuel.  less energy per unit volume implies that ALL ELSE EQUAL less
: power will be produced by the engine running on high octane than running on low
: octane.  this assumes that the engine management computer system doesn't
: compensate for fuel octane, by the way...
: 
: walt k.
: 

This is NOT TRUE!!!!  We run our engine tests on three grades of fuel; 91 RON,
97 RON, and 103 RON. The lower heating value is within 1% for all three fuels.
-------------------------------------------------


Now as to a simplified explaination of the importance of octane.

First, for a given engine design (valve timing, comp. ratio, etc), torque at any
operating condition follows a roughly parabolic relationship with spark timing.
That is, for a given condition there is a single spark advance value that 
provides maximum torque. The location of the peak is refered to as MBT (Minimum 
spark advance for Best Torque). The location of the parabolic curve shifts with 
load and speed. In an ideal world, one would always calibrate the ignition 
system to be at MBT for the whole speed/load map. If we neglect emission 
constraints and have higher enough octane fuel, then all engines would be 
calibrated to run at MBT.

The problem is that almost all things done to increase thermal efficiency/power 
tend to exacerbate the octane requirement of an engine. In addition, octane requirement changes with age (deposit formation) and operating environment. 

With these effects in mind engine designers attempt to design the engine to be
some compromise between efficiency, power, emissions and octane requirement.

So lets look a a couple of really simplified examples.

Engine 1.
   Design choices 
     91 RON Fuel (87 pump rating)
     good torque at low speed ==> 8.5:1 CR and short valve events
     no knock sensor

Engine 2.
   Design choices
     91 RON Fuel (87 pump rating)
     high speed power + good torque at low speed ==> 9.0:1 CR & longer valve 
     events
     knock sensor (knock limited with 91 RON fuel at low speed)

Engine 3.
   Design choices
     91 RON Fuel (87 pump rating)
     high fuel economy with OK power/torque  ==> 10.5:1 CR & moderate
     valve events
     knock sensor (inherently knock limited at WOT for all speeds)

What happens at WOT if you use high octane in each of the above engines? As
a reminder, assume the knock sensor only retards from true MBT when it "hears"
knock. 

Engine 1: Nothing, you waste your money. Why? Because the engine doesn't need
          the higher octane fuel. It's already running at MBT. If age leads to
          unacceptable knock, then the higher octane fuel will quiet the knock.

Engine 2: Increased low speed torque. Why? Because the engine couldn't run
          at MBT at low speed. This was one of the design compromises. Give up 
          low speed torque to get good high speed power. (Octane requirement tends
          to diminish as speed increases). The control system can now allow spark
          to be set to MBT at low speed.

Engine 3: Increased torque at all speeds. Why? Because the engine couldn't run
          at MBT at any speed. Now, the control system will allow MBT. Fuel economy
          might increase if the engine spent much time at moderate to high loads.

So what's the bottom line?

It depends on how well the engine was designed and how well it was calibrated.

There is no fixed relationship between octane rating and power.
 




--
*************************************************************************
*My opinions are mine and do not necessarily reflect in any way those *
*of my employer.                                                        *
*Chris Warren:  ccw@slee01.srl.ford.com                                 *
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