05-26-2007, 12:44 PM
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Active Enthusiast
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Hybrids: Toyota Highlander
Posts: 80
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Re: Premium gas...?
Quote:
Originally Posted by chicagoandy
The HyHi manual states that it requires at least 87 Octane gas, but higher octane is preferred.
I've been running 87 Octane only, since I'm a cheapskate.
I've always understood that if you're not hearing engine-knock, then there's no reason to buy premium gas... But I think this gets more complicated with computer controlled variable timing engines. Does this old rule still apply?
And more specifically, is there really any reason to run high octane gas in the Hybrid Highlander? Will it increase my mileage? Will it increase my mileage enough to pay for the more expensive gas?
Thanks,
_Am
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Lower compression engines that are designed for 87 octane only will not benefit from premium and will actually get lower MPG and less engine power with premium. Also, using premium in such low compression engines may contribute to carbon deposit buildup.
However, Toyota has designed the Highlander Hybrid engine with a very high compression engine to take advantage of the properties of premium fuel if it is used. Toyota recommends premium fuel in the owners manual for best performance in the HiHY. Note that using 89-93 octane in the Highlander is optional. Using 87 octane will not cause any drivability problems.
If you use premium in the Highlander Hybrid, the engine will be more much more responsive as expected. However, the engine will also run more efficiently when driven conservatively such as when you are driving with cruise control on at highway speeds. The engine actively advances the ignition timing as much as allowed by the fuel grade used under light engine loading and part throttle to maximize fuel efficiency.
I have a Scan Gauge II in my Highlander and it digitally displays the engine timing advance every few seconds. I have noted that the engine is at it’s most advanced timing settings when driving down a level highway at 65mph with cruise control on and the engine at a low and stable RPM. 89-93 octane will lead to improved fuel economy if you have the self-control not to use the added reserve horsepower.
The other part of the equation is the differential in price between regular and premium. When gas was $2, 91-93 octane was typically $0.20 more or a 10% surcharge over 87 octane. With gas closing in on $4 per gallon, premium is still only typically $0.20 more resulting in a reduced 5% surcharge over regular. Thus the percent cost difference between premium and regular is narrowing considerably as fuel prices rise. This may tip the balance for the premium’s extra fuel efficiency paying off. With a typical 28 MPG for the Highlander, you need to improve fuel mileage with premium over regular by 1.4 MPG to break even. When gas was $2/gallon, A 2.8 MPG improvement was needed to break even.
The Highlander Hybrid has a very high compression ratio of 10.8. This predisposes it for higher octane fuels. High compression ratio engines are more efficient than low compression ratio engines all else being equal.
However, the higher pressures in the cylinders cause the gasoline to be more difficult to control from a combustion standpoint. Gas will ignite more rapidly at higher cylinder pressures which can result in engine knock if the combustion peak pressure occurs too early in the downward stroke of the piston. Engine knock can damage an engine.
Modern engines all have knock sensors tied into the vehicle’s engine controls which operate with lightning speed to retard (delay) the ignition spark timing to head off this knock condition before the driver could ever notice it. This automatic dial back of engine timing and engine efficiency is what allows 87 octane fuel to be used in the Highlander’s high compression engine without any adverse effects.
Premium fuel requires higher temperatures and pressures to combust properly and it burns more slowly than regular fuel. Thus, premium fuel behaves more controllably under the higher compression conditions in the Highlander engine and will allow the engine to advance the spark timing more than with regular fuel resulting in higher engine efficiency.
The standard internal combustion engine is terribly inefficient. Only about 30% of the gasoline you buy is actually converted into mechanical energy to make the vehicle move. A whopping 70% of the fuel you pay for is just converted into wasted hot air that escapes from the radiator, the tailpipe, and radiates off the engine itself.
The engine ignition timing is extremely important in extracting the maximum amount of mechanical energy from the burning fuel. It’s a complex choreography but in simple terms the fuel burn should be timed to reach maximum piston force at approximately 15 degrees of crankshaft rotation past top dead center of piston travel.
When lower octane fuel is used in a high compression engine, the fuel burns more quickly, reaching peak forces much sooner than high octane fuel. This potentially causes knock. The spark timing is retarded to reduce the knock but this results in less of the fuel energy being captured by the piston as mechanical energy and more being ejected from the engine as waste heat.
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