I thought I was the only person to own an 87 CRX HF. That thing could drive all week on $10, in late 90s gas prices.
I drive a Subaru WRX with a similar style engine, turbocharged fourbangers are becoming commonplace on cars these days and in them, the gasoline grade absolutely makes a noticeable difference.
My experiences pretty much line up with qazwart’s entirely.
Compression ratio does not affect combustion efficiency (defined as % of fuel’s chemical energy converted to heat/pressure during a combustion event). Neither does octane ratio. In a modern gasoline spark-ignited engine, combustion efficiency is upwards of 95%, regardless of octane or compression ratio.
Compression ratio (more specifically, expansion ratio) DOES affect how much of the available thermal energy (manifested as heat and pressure in the combustion products) gets converted to mechanical work by the movement of the piston.
Retarding the spark (from max brake-torque spark timing) results in reduced efficiency. Power output does decrease because of this, but only indirectly.
I haven’t seen anyone else mention this, but octane ratings are also elevation dependent. So for those who live above about 4000’ you should generally be using a octane 2 points lower than those who live in the lowlands. So if your vehicle recommends a 87 octane, you can and should use the 85 octane safely.
Just a note - Europe uses a different way of measuring octane ratings than North America. Europe uses the Research Octane Number (RON) while North American measures octane ratings using the Anti-Knock Index (AKI) which is the average of the RON and the Motor Octane Number (MON). Generally, the AKI number will be 4-5 points lower than the RON for the same exact fuel, so the fuel you get in the UK would be rated at 90-91 and 92-3 or so in the US.
This woud be true only in a car lacking a turbocharger or supercharger. Many modern cars have them. Particularly those bought by people who live at altitude.
Higher octane gas WILL provide more power in many modern cars. Not because the gas is providing the extra power, but because modern cars are designed with higher compression and other power enhancing modifications that require higher octane. Failure to use the premium fuels in these cars will result in loss of power (11 HP in the 2011 Mustang GT. per Ford’s claim) to other problems as severe as engine damage.
Actually, this isn’t true for virtually modern car, whether it uses compressed or naturally aspirated air. All modern cars with electronic fuel injection will advance or retard the spark to obtain most efficient combustion for a given oxygen level and fuel combustion threshold. Many modern cars also use variable timing on the valve opening to enhance combustion. You should not try to second guess the optimal operation of the engine from vaguely understood principles; the operator’s manual specifies what AKI or RON rating of fuel to be used. This will provide optimal performance in any factory-standard motor. Any more or less will generally offer poorer performance, waste of money, or damage to the engine.
There is no good answer to this. The AKI or RON number says nothing about what the energy density or process efficiency is for the fuel. It only offers a relative measure of the threshold for combustion. Higher compression engines obtain more power per cycle by using high octane fuel (allowing the engine to combust more fuel at once or at faster cycle rates) but the energy content of the actual fuel may be lower than a lower RON fuel.
Stranger
Thanks, I think I understand. The compression ratio of an engine is fixed, but the amount the fuel-air mix is actually compressed at the point of ignition is changed slightly by the timing. Do I have that straight?
Looking at my car, rated to run on 95 Octane, I’m still not sure if it will actually run better on 97 Octane. It must depend on exactly what the compression ratio is. My guess is the engine is designed with a slightly higher compression ratio than fuel it’s rated for. The engine’s peformance may differ It may also depend on variables such as the air temperature. (I’m asking out of curiosity, even if 97 is better it’s not worth the cost).
Thanks for clearing that up, I thought that was a bit strange.
The spark timing doesn’t change the amount of fuel-air mix or the compression ratio.
Some cars have variable valve timing. That can affect the amount of fuel-air charge.
My owner’s manual says “87 or higher.”
Almost. The mixture is compressed by the piston’s action, and it’s the same every time because the piston and cylinder geometry is the same. But the timing – when the spark is ignited – can be adjusted to be a little early or late based on what the computer thinks is the best.
With older engines, you could advance or retard the spark manually. You would want to advance the spark (make it fire earlier) if the engine was rotating faster to compensate for the very small, but detectable delay from the time the spark was initiated until the actual explosion. As the engine goes faster, if you don’t adjust this, the fixed delay time translates into a later actual time in the cycle, since the cycle time is getting shorter.
In other words, in a faster engine, the cylinder gets to ideal firing point X before the spark does if you don’t compensate by firing a little sooner.
Reading this back, it might not be as clear in your mind as mine. There are probably some animations somewhere on the WWW that you can find.
The key is you want maximum cylinder pressure when the piston is at a certain point in its cycle. This is so that the maximum rotational energy is “harvested” from the pressure built in the cylinder. If my aging memory serves, this is usually around 7 degrees after top-dead-center (ATDC).
It takes time for cylinder pressure to build after the spark plug has ignited the mixture, so the plug has to fire much earlier than 7 dgr ATDC. At idle you can see spark timing of 20 dgr BTDC (Before top-dead-center). So even at idle and the engine turning slow, it can take 27 dgr of crankshaft rotation from the time the plug fires until maximum pressure is achieved.
The optimum time for the plug to fire changes with RPM as well as load, but you always want to reach peak pressure at the same point (7 dgr ATDC).
Now for octane: Some of the posts above give the impression that running fuel with insufficient octane makes it ignite before the plug fires. That is not correct. If fuel ignites before the plug fires it is called “pre-ignition”. An engine will self destruct in very short order (a matter of seconds) if it experiences pre-ignition.
Pinging (not pre-ignition) is what you get with fuel that has insufficient octane. Pinging is when the plug ignites the fuel as it should. A flame front spreads from the ignition point and pressure in the cylinder starts increasing. However, due to the low octane, fuel in another part of the chamber spontaneously ignites due to the rapidly raising pressure and creates a second flame front. The pinging noise is the two flame fronts colliding, ringing the engine block like a bell.
Pinging is not particularly harmful. It’s certainly not good for the engine, but you need to drive for quite a while before you would see damage.
BTW, I keep seeing the term “explosion”… there is no explosion. It is combustion. The distinction is important as combustion is a relatively slow process compared to an explosion.
In that case you should run 87 to get maximum economy & power. You will see no benefits with higher octane (although higher octane will not harm the engine).
If the engine is designed to take advantage of higher octane the wording will be along the lines of “91 or higher recommended”. That means max power & economy will be achieved with 91 octane fuel. Lower octane can be used without damage, but with a reduction of power & economy.
From the OP:
No one’s addressed this yet (and it might not be an actual question the OP had), but fuel additives almost certainly cost much more per additional miles obtained than any gasoline you might buy.
For clarity’s sake: “explosion” is a vague/general term that refers to any buildup/release of energy, whether from combustion, mechanical disintegration, pressure-vessel rupture, or whatnot.
Where chemical reactions are concerned, depending on the circumstances you can have two very different processes:
Deflagration. This is a combustion event in which the reaction front moves through the reactants at subsonic speeds. The “laminar flame speed” of a flammable mixture is the speed at which the flame moves through a quiescent mass of reactants. For gasoline-air mixtures, it’s on the order of a foot per second. This is actually painfully slow, a speed you can easily walk away from on foot. The good news is that turbulence speeds things up quite a bit; and since turbulence scales with engine RPM, this is why it’s possible to run an engine at ridiculous speeds, like 16,000+ RPM.
Detonation. This is a combustion event in which a shock wave passing through the reactants is what initiates the reaction itself; the result is a reaction front that moves through the reactants at supersonic speeds, on the order of miles per second. When your engine knocks, the “pinging” you hear is shock waves (spatially discontinuous rises in temperature, pressure, and density) reverberating back and forth in the combustion chamber. Based on the size/shape of the combustion chambers in an engine, there is a characteristic audio frequency associated with this “pinging,” and that’s what a knock sensor is listening for. Detonation is bad for your engine because where those shock waves hit the cylinder wall, they result in extremely high pressures and temperatures for a brief instant. Light or intermittent knock won’t hurt things, but if it keeps up for significant length of time, the elevated temperatures and pressures can erode head gaskets and dump significant amounts of heat into the cylinder wall, eventually overheating/damaging your engine.
When a mixture of gasoline is ignited, the already-burned part expands and ends up compressing the unburned part even further, driving it to higher pressures and temperatures than the first part. If the mixture is held at high pressures and temperatures for some length of time (milliseconds), it will autoignite. If the initial pressures and temperatures are high enough, the deflagration may transition to detonation, in which case you’ll hear knock/pinging. It is this transition to detonation that higher-octane fuel is more resistant to, and it’s a time/temperature thing: higher octane buys you more time to complete the combustion event before the transition to detonation happens (and/or it resists autoignition/detonation at higher temps). If you can complete the normal burn fast enough, detonation won’t happen. If you can’t, well, then you need higher octane to resist detonation and buy you more time for that normal burn.
Some fuel-air mixtures may detonate under normal atmospheric conditions. Oxyacetylene is a good one; sometimes if you get the mixture set wrong on an oxyacetylene torch, the flame will burn out with a loud, sharp SNAP; that’s detonation. Hydrogen-oxygen mixtures can detonate, too. The first explosion at the Fukushima nuclear power plant in Japan, caught on video, is clearly a detonation; the shock wave propagating upward from the blast is evident. A natural-gas-and-air mixture may blow a house apart due to rapid deflagration, but it won’t detonate like a hydrogen-oxygen mixture will.