Newsletter No 47

 

13 August 2007

 

Hi. There

 

I am still struggling with my PC. The latest software does not want to interact with my website.

I feel a bit silly to send out a Newsletter without doing urgent updates on my Website.

 

But as threatened in my last Newsletter, this time I’ll concentrate on Compression Ratio vs. Boost vs. Timing vs. Octane rating.

Before I start on this I’ll give you the Wikepedia explanation.

1. intake/induction stroke
2. compression stroke
3. power stroke
4. exhaust stroke

Four-stroke cycle (or Otto cycle)

The cycle begins at top dead center (TDC), when the piston is furthest away from the crankshaft. On the first stroke (intake/induction) of the piston, as the piston descends it reduces the pressure in the cylinder, a mixture of fuel and air is forced, by at least atmospheric pressure, into the cylinder through the intake (inlet) port. The intake (inlet) valve (or valves) then close(s) and the following stroke (compression) compresses the fuel-air mixture.

The air-fuel mixture is then ignited, usually by a spark plug for a gasoline or Otto cycle engine or by the heat and pressure of compression for a Diesel cycle or compression ignition engine, at approximately the top of the compression stroke. The resulting expansion of burning gases pushes the piston downward for the third stroke (power) and in the fourth stroke (exhaust) the piston pushes the products of combustion from the cylinder through an exhaust valve or valves.

Now all I am interested here is creating as much power on the crank as possible utilising the Power stroke:

I hope you agree that you want the Gasoline mixture on top of the piston to start making pressure not too far before it reaches TDC.

Also it should stop making pressure just after the piston reaches BDC.       At 7000 Rpm that will be within 0.004286 seconds.

The spark is started before TDC as we have to take delays into account. Starting it to early will give us pressure on the Piston before it reaches TDC. Starting it to late will let the piston go down without exerting meaningful power on the Crankshaft.

 

The more mixture you can “squeeze” on top of the piston, the more power you will make. So you increase the Compression ratio. Either Mechanically (Compression Ratio, Cam Timing, Exhaust puling etc.) or by adding a Turbo or chemically such as NOS.

Of course there comes a point where you get Detonation. Again by Courtesy of Wikepedia:

Knocking (also called pinking or pinging)— colloquially detonation—in internal combustion engines occurs when air/fuel mixture in the cylinder has been ignited by the spark plug and the smooth burning is interrupted by the unburned mixture in the combustion chamber exploding before the flame front can reach it. The engineered combusting process ceases, because of the explosion, before the optimum moment for the four-stroke cycle. The resulting shockwave reverberates in the combustion chamber, creating a characteristic metallic "pinging" sound, and pressures increase catastrophically.

Detonation can be prevented by:

• The use of a fuel with higher octane rating
• The addition of octane-increasing "lead", methylcyclopentadienyl manganese tricarbonyl (MMT), isooctane, or other antiknock agents.
• Increasing the amount of fuel injected/inducted (resulting in lower Air to Fuel Ratio)
• Reduction of cylinder pressure by increasing the engine revolutions (lower gear), decreasing the manifold pressure (throttle opening) or reducing the load on the engine, or any combination.
• Reduction of charge (in-cylinder) temperatures (such as through cooling, water injection or compression ratio reduction).
• Retardation of spark plug ignition.
• Improved combustion chamber design that concentrates mixture near the spark plug and generates high turbulence to promote fast even burning.
• Use of a spark plug of colder heat range in cases where the spark plug insulator has become a source of pre-ignition leading to detonation.

Pre-ignition is a different phenomenon from detonation, explained above, and occurs when the air/fuel mixture in the cylinder (or even just entering the cylinder) ignites before the spark plug fires.

 

So to get the most Power we need to have the correct timing, as well as the correct AFR. There is only a correct value and any rocking about because of other faulty Engine parameters will do nothing for you except loose Power.

As you can see there is no Magic.

Create the Maximum allowable pressure on your Piston ad you make the most Power.

As you can see above to avoid detonation you will have to use a Fuel with a higher Octane ratio.

Enter Race Gas which does have an octane rating higher than pump fuel but lower than Avgas.

Avgas burns too slow though.

 To retard timing to avoid Detonation just wastes Power. Therefore be aware of excessively skimmed heads.

 

Right now I am up to tuning 2 Turbocharged racing vehicles. All this comes into play then J

Beginning of September will find Urda & me in Lubango for the races and hopefully a few days of R&R afterward.

By then I hope my Website/Newsletter will be sorted out again.

 

But if you have anything that needs to be done in the meantime, just let me know.

 

Thanks for Bearing with me

 

Nick

 

"SUCCESS is neither magical nor mysterious. SUCCESS is the natural
consequence of consistently applying BASIC FUNDAMENTALS"    Jim Rohn