Friday, April 25, 2014
Octane Number- is that like a phone number?
You notice the fuel gauge on your car is getting low and your favorite fuel station is just up ahead, so you stop in to fill-er-up. Do you choose the best fuel for your vehicle, or do you buy based on cost? Most fueling stations have 3 different grades of gasoline- have you ever wondered why?
Have you experimented with different grades of fuel to see which your car likes? No? Most folks I have spoken to, or simply observed at fueling stations, just buy the cheap stuff and go on about their day feeling like they got good value by paying less per gallon.
Simply put, gasoline engines are all built differently, for different purposes, with slightly different materials and design criteria for specific applications. The engine math used to design it may be radically different from one engine to another. Length of stroke, diameter of the bore, compression ratio, flow through intake and exhaust systems, ignition timing (all engine math variables...), as well as balancing and basic design play a major role in engine performance. The burn characteristics of the fuel used in said engine play a crucial role in final power production and efficiency as well.
Combustion in an engine is a moving flame front through the fuel and air mixture that happens extremely rapidly- almost explosively. By the standards of our human senses, it is explosive in nature, but technically combustion in an engine is NOT an explosion, it is a controlled rate burn. The higher the compression ratio of an engine, the faster the burn in the cylinder because the oxygen and fuel molecules are crammed tighter together. This rapid burn generates tremendous heat, therefore tremendous pressures in the combustion chamber. It is this pressure that pushes the piston back down to do work.
The classic diesel 'knock' or 'clatter' is the sound of the fuel detonating so rapidly that the shockwave actually rings the entire engine block like a bell. Diesel compression is typically about double (20:1) that of standard gasoline engines (10:1) and thus the speed of burn is also much faster in diesels. Compression generates heat- simple rule of physics- as a gas get compressed, its volume decreases and the temperature and pressure rise proportionally. A diesel engine makes enough heat from compression that when the fuel is injected into the combustion chamber, it ignites immediately.
A gasoline or CNG engine takes its fuel in with its air, compresses them together and then ignites them with a spark at the optimal moment (ignition timing). If the fuel ignites during compression instead of waiting for the spark (detonation, ping, knock) not only do you lose power but counteracting pressure waves of compression and combustion combine to drive cylinder pressures far above design parameters and can break, bend and wreck parts right meow. Ignition timing is crucial to engine performance; uncontrolled ignition is unacceptable in any form- whether from carbon stuck on the piston or head and glowing hot, worn or broken timing components, slipped timing belt, low octane fuel...
What I am getting at here is that fuel for gasoline engines must ignite easily with a spark, but must wait to ignite through the entire compression stroke until that spark. This ability to resist compression ignition- even when the throttle is wide open, volumetric efficiencies and observed compression pressures are highest and thus compression temperatures are also maxed out- is the important part of 'Octane rating' The higher the apparent octane number of a fuel, the higher the compression and temperature that fuel can handle without self ignition. Higher octane fuels become more necessary as the engine spends more time at full throttle working hard, or when greater compression ratios are designed into the engine.
Many vehicles with low initial compression or vehicles with huge, overpowered engines (think 1970 Cadillac DeVille) since they rarely need full throttle for more than a few seconds in normal use, can safely run low octane fuels. A race-built or small engine that can spend a large majority of its time at very high power outputs must have high octane premium fuels to retain decent power, keep combustion and engine temps in the safe zones, and prevent immediate catastrophic engine damage when the throttle is opened under high load. Motorcycles, snow-machines, airplanes, chainsaws, ATV's, and other small, high performance engines must use high octane fuels at all times for engine performance and longevity.
High octane fuels (91 octane up here at high altitude, 94 or 95 octane at sea level) burn a bit more slowly and controllable than their low octane brethren. Therefore, the ignition timing can be advanced in the cycle to allow more time for as much of the fuel/ air mixture to burn as possible, therefore extracting the maximum heat and energy from that fuel. The burn happens first, immediately after the spark, then the heat and pressure climb rapidly just behind the flamefront.
Getting the combustion, heat and pressure to all occur at the proper times requires some tweaking, and for most engine manufacturers that means not being to aggressive about compression, timing or fuel requirement and going instead for wasting a bit of fuel in trade for consistency and simplicity- for the engineers sake and for the operator of the machine. This trade of simplicity for performance reduces overall efficiency and fuel economy, but makes a smooth, wide powerband (the RPM range where an engine makes the most power) and very useable and driveable engine performance curves.
In most cases, power, efficiency and temperature curves are all most optimal when the spark is advanced to the edge of ping at wide-open-throttle using a fuel that has the correct burn characteristics for the compression ratio of the engine. No, there is no rule of thumb on this, except that if it knocks or pings at full throttle, retard (delay) the spark a bit, or increase the octane rating of the fuel. If you can advance ignition timing a bit, you will need higher octane fuels.
In performance only applications, I do not use a timing light to set ignition timing, I use only my ear. When tuning a vehicle for power and/or fuel economy, I want the the engine to pull its hardest on premium fuel, and have the correct sound with only minor ping under hard acceleration. The point that those criteria overlap is your optimal ignition timing for that fuel. If low-octane fuel were then substituted into that high performance engine tuned for premium fuel, it might destroy the engine the first time you stuffed your foot into the skinny go pedal.
On the distributors of most vehicles I have owned over the years, I have used a small pick to scribe 'octane marks' AKA- 'altitude marks', onto the distributor to line up with altitude or fuel changes. Drop altitude or need to use low octane fuel? No problem. Loosen and rotate the distributor to the correct scribe mark- its that easy.
Computers control timing nowadays, with 'knock sensors' to be able to hear that knock or ping and retard timing appropriately. Motorcycles and other gas-powered toys, sportscars, souped up cars and trucks, and most two-cycle engines need high octane premium fuels for longest engine life and maximum performance. This is not a choice, its required. I use octane boosters in my small engines to help keep temperatures down, burn rates smooth and slow, and maximize power output and fuel economy.
If the engine pings going up a hill, use higher octane fuel, until then, don't let it ping- back off the accelerator.
Use what makes the engine happy and most efficient; if the engine is happy with lower octane fuels, fine.
Some engines see gains in MPG with higher octane fuel, some don't. Experiment to see.
Generally, when you advance ignition timing, you get more power and better MPG, but you need to use higher quality /price fuels.
Cost/ benefit analysis is needed to determine savings. Most high performance engines or small engines working hard all the time need premium fuels to maximize engine life and control heat build up.
Got a question, comment or thought on this? Then speak up!! Use the comment section below to speak your mind. Till next time...