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Discussion in 'Chevrolet Volt' started by Former Member 68813, Apr 23, 2014.
Higher compression ratio? You're confusing the above with the high expansion ratio, not compression.
There is no need for premium in a Prius, unless you like wasting money.
Additionally, premium has a lower energy content than regular unleaded.
Additionally, premium has a lower energy content than regular unleaded*.
*I must be showing my age by calling it "unleaded." Leaded gas hasn't been around in quite a while.
How do you explain your statement about energy content?
I thought gas was gas and only the octane enhancers are different.
Of course 10% ethanol content lowers the BTU/Gal content of gas.
Also, 100LL (100 octane Low Lead) Aviation Gas is available at most airports that handle General Aviation.
It's the only gas for most piston powered planes. Some smaller planes have a legal mod that allows Car Gas.
There is current news that once again the EPA wants all the lead gone.
Looking into it, I can't find a good source stating premium versus regular, besides the fact that companies may add ethanol to the mix to increase octane rating. Since ethanol has a lower energy content than gasoline, that would lead to an over all lower energy content. It may be negligible compared to variations between brands anyways. So never mind.
Premium can have lower energy content, but it is small. Something like a few hundred btus between fuels with 30k+ btu/gal. Then the differing blends could widen or narrow that difference.
High compression engines can be more thermally efficient than one with a compression stroke for regular. So even if the premium fuel has less chemical energy, the engine converts more of it into mechanical energy. Giving the system an overall lead in comparison.
Good catch on the lower energy density on the premium. I totally forgot about that one. Also the high octane has an increased resistance to engine knock. Of course, in todays' modern engines, knock sensor(s), working with the emission systems adjust the timing to prevent knocking.
Even so, a gasoline engine converts from chemical to mechanical at the same efficiency, be it regular or premium gas.
There is not increased in efficiency using premium over regular.
The majority of todays' engines (Volt excluded) are happy and perform quite well on regular gas.
Bill Norton Says:
That depends on A/C age. 1970's era use E0 avgas. Engine manufacturers requirement forbids E10 in these A/C.
Can that be backed up?
The higher octane allows a higher compression ratio. The flip side of the compression stroke is the expansion or power stroke. The higher the ratio, the more time the burning air and fuel mixture has to push down on the piston.
An engine with knock sensors can run lower octanes. It manages this by retarding the ignition timing. The ideal time for ignition is just before the piston reaches the top of the piston. There is a slight delay between when the spark plug fires and the fuel mixture really starts burning. This allows the burning gases in the cylinder to start expanding and pushing down on the piston at the top of the power stroke.
When ignition timing is retarded, the spark plug fires at a time later than ideal. So the pushing from the expansion of the fuel happens after the piston has started moving down. It is in effect chasing the piston down while energy from previous power strokes is being wasted pulling the piston down.
Ignition timing - Wikipedia, the free encyclopedia
HowStuffWorks "Ignition System Timing"
Most full hybrid powertrains use an 'Atkinsonized' Otto Cycle gasoline engine that use modified valve timing to increase the effective expansion ratio of the combustion. This increases efficiency, in part, by lowering pumping losses but results in reduced power density (same power output requires a bigger displacement engine).
The Volt uses an existing Otto Cycle engine that GM uses in European cars. Conventional cars control the throttle plate position to control engine power output and RPM. GM typically runs the engine in the Volt with a wide open throttle to reduce pumping losses and loads down the engine RPM by generating electricity with the Volt's smaller electric motor.
Engines running under load with an open throttle like this are prone to engine knocking which is presumably why the Volt is specified to use higher octane premium fuel.
Engine knocking - Wikipedia, the free encyclopedia.
So yes, there is actually a reason why using higher octane gas in a Volt causes it to operate more efficiently. If you put in regular gas the knock sensor may cause the engine controller to partially close the throttle and reduce the generator load thereby increasing the pumping loss.
We don't know why GM chose this path which implies premium fuel rather than using an Atkinsonized engine. One possible answer is that the engine power output they needed would have required them to use a larger displacement engine. For example, the Volt uses an Otto Cycle 1.4L engine where the Prius uses an Atkinson Cycle 1.8L engine.
One possible answer is that they could not mechanically fit the GM 1.8L engine under the hood along with the other components like the transaxle and motor power electronics given the limitations of the existing platform they were building on.
I once had a similar experience with a rental car. Except the key was in my front pocket, so I guess you would call that, uh, ...nevermind. Anyway, I parked directly in front of a restaurant, and when I came out after dinner, the trunk was wide open, and our suitcases were being rained on. But fortunately they were still there. The key fob had a trunk opener button that was barely recessed, and easily accidentally pressed.
Getting back to the main topic, looking through the reported repairs, the most important thing is what is not there. There were no major problems specific to the hybrid system. For this I congratulate GM and the Volt buyers who persisted despite the naysayers.
Speaking for myself, I could live with most of the relatively minor annoyances of the Volt. Naturally, since the Prius is one of the most reliable cars in the world, the Volt doesn't look so good in comparison, but there are others that are worse.
Government Motors bean counters had final say in the Otto cycle engine in the Volt. Use up existing supply of engines for the Volt.
Had there been deeper pockets to draw from, Volt would had been better suited to the Ackerson - Miller cycle engine.
Higher compression requires the use of premium, high octane gas. This is a no brainer. High resistance to knock is premium claim to fame.
A very simple, old, trick GM used on the Volt ice - increased power by increased compression ratio. This mandates the use of premium, there is no way around this using the current crappy ice in the Volt.
Nope. The Volt's 1.4L engine has a compression ratio of 10.5 just like Toyota's 1NZ-FE 1.5L Otto Cycle engine used in the Yaris and various other conventional cars that run on regular.
Toyota NZ engine - Wikipedia, the free encyclopedia
Likewise, I'm pretty sure the conventional non-hybrid European Opel models that use the same engine as the Volt do not require high octane gasoline.
The Opel Astra with a nearly identical engine is recommended to use what would be considered mid-grade gas in the US (95 RON) probably because this is the "regular" gas in many European countries. GM/Opel also says 91 RON (US regular) and 98 RON (US premium) are also fine.
Contrast this with the Volt where GM "requires" premium and recommends against lower octane gas except when premium gas is unavailable. Interestingly, the European version of the Volt (Opel/Vauxhaul Ampera) also recommends 95 RON (US mid-grade).
The likely reason the Volt specifies 95 RON or higher octane gas is because it frequently runs the engine at wide open throttle under load from the smaller motor generator. That reduces "pumping losses" and increases engine efficiency in a way that is not normally possible on a conventional car but the open throttle and high load makes engine knocking more likely at an otherwise
conventional compression ratio.
In my earlier reply to you there is a link to a Wikipedia article on engine knock which discusses the issues with open throttle and load.
Bad news - running the ice @ full throttle. An engine suited / designed correctly for providing power to run the generator does NOT need nor should be designed to run at full throttle at all times.
But, then again, this is the engine that the bean counters mandated to be used in the Volt.
Why? legacy will show decreased ice life in the Volt.
But, what do you care, if you are leasing a Volt. It's not a real concern for you, when at lease end, you turn the car back to the dealer.
However, for legacy sake, it gets to be shown, the Volt ice lives a short life.
This des not bode well foe the Volt building a reputation for a long life.
Why don't we focus on the facts rather than speculations?
DB, why do you think the Volt runs its engine at "Full Power". I don't see this ever on mine.
You may be reading 5 year old articles about the Volt.
Also if my volt at total 16k miles only has 5k on the engine, how will I know and when will I know, "the Volt ice lives a short life."?
The engine has only run about 15% of the time in my driving. I'm pretty sure not running the engine will help it last longer. At my current pace, I'll have to do my first oil change after 2 years (and I drive 18,000 miles a year).
I guess we'll see.
On oil changes, GM's oil life monitor(OLM) is probably the best in the business. It is actually calculating the usage rate of a common additive(anti-wear I think) based upon engine speed, load, and temps. A Prius with such a system should easily see oil change intervals in the 8k to 10k range.