When did diesel start being cheaper than 87 octane? (gasket, idle, exhaust)
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You are exactly right. Ethanol has lower energy content per gallon than does pure gasoline. So, burning higher octane fuel in a car that doesn't need it, when the octane rating is raised by adding ethanol, will result in lower fuel economy. Burning higher octane fuel, even when it is pure gasoline, provides no real benefit to a car designed to run adequately on lower octane fuel. By the way, at high-altitude (above 4,000-5,000 feet elevation), 85 octane fuel will perform equally well as 87 octane regular gasoline in vehicles designed for regular gasoline. All my driving is at high-altitude, and I have used nothing but 85 octane fuel in my gasoline vehicles for decades, with no ill-effects on engine performance or longevity.
That all used to be true, but largely is not any more. Old naturally-aspirated engines with fixed valve and ignition timing can run acceptably at high altitudes with lower-octane fuel since the effective compression ratio drops with the decreasing atmospheric pressure. (Higher elevation = fewer air particles per unit volume = lower pressure/compression.) Timing was set to not be too advanced under heavy loads and compression ratios were also pretty low at around 9:1. This was fairly forgiving even if your octane was a little lower than really needed, you were still not going to knock much unless you hammered the throttle. Putting anything more than their rated octane was a waste since they would run fine on anything that met the minimum octane...and then had no means to alter their operation to take advantage of higher-octane fuel.
Modern engines have compression ratios in the 10-12:1 ratio, multiple knock sensors and variable valve and ignition timing. These engines are really optimized for 89-91 octane fuel and make heavy use of knock sensors to retard ignition timing and have conservative valve timing to not knock when using 87 octane regular. Putting higher-octane fuel in a modern engine often WILL cause it to perform better because the engine will stop retarding ignition timing and valve timing. For example, Ford's new truck 5.0 L V8 makes 360 hp on 87 octane and 375 hp on 91 octane premium. You can get by with running more advanced timing at high altitudes when using lower-octane fuel than at sea level in the old days due to lower effective compression ratios at high altitudes. However there isn't much wiggle room with newer high-compression, aggressively-timed engines and you need to really be careful with your elevation and your octane. You may actually need 86 octane at your particular elevation to not knock rather than the 85 sold in the pumps.
Turbocharging is common on newer engines and you absolutely CANNOT run low-octane fuel in them. The turbo will pressurize intake air to the exact same pressure at any elevation. You get the same amount of air particles in your cylinder at any elevation and you will need the exact same octane at sea level as in the mountains. To add insult to injury, turbo engines are also much more likely to require higher than 87 octane fuel than naturally-aspirated engines because the turbo's boost increases the effective compression ratio of the engine to much higher than that of a modern naturally-aspirated engine. High elevation areas also tend to be hilly so you are really loading up your engine driving up the steep grades.
So in short, I would not recommend running anything less than the rated octane in any reasonably modern engine at any elevation unless you want to risk grenading your $4000 engine to save maybe two bucks a tank in fuel costs.
Originally Posted by jazzlover
By the way, at high-altitude (above 4,000-5,000 feet elevation), 85 octane fuel will perform equally well as 87 octane regular gasoline in vehicles designed for regular gasoline (emphasis mine). All my driving is at high-altitude, and I have used nothing but 85 octane fuel in my gasoline vehicles for decades, with no ill-effects on engine performance or longevity.
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Originally Posted by Flyover_Country
That all used to be true, but largely is not any more. Old naturally-aspirated engines with fixed valve and ignition timing can run acceptably at high altitudes with lower-octane fuel since the effective compression ratio drops with the decreasing atmospheric pressure. (Higher elevation = fewer air particles per unit volume = lower pressure/compression.) Timing was set to not be too advanced under heavy loads and compression ratios were also pretty low at around 9:1. This was fairly forgiving even if your octane was a little lower than really needed, you were still not going to knock much unless you hammered the throttle. Putting anything more than their rated octane was a waste since they would run fine on anything that met the minimum octane...and then had no means to alter their operation to take advantage of higher-octane fuel.
Modern engines have compression ratios in the 10-12:1 ratio, multiple knock sensors and variable valve and ignition timing. These engines are really optimized for 89-91 octane fuel and make heavy use of knock sensors to retard ignition timing and have conservative valve timing to not knock when using 87 octane regular.
Flyover ...
jazz clearly qualified the use of the lower octane fuel to the vehicles designed to run on it, which is most of what you'll see on the roads around here.
While latest gasoline engine technology has us seeing "10-12:1" and as high as 14:1 direct injection gasoline engines now coming into production ... with all the latest technology to capture the efficiency of a higher compression gasoline engine ....
they are a miniscule portion of the vehicle fleet on the road today, probably less than 1% of the cars on the road.
As well, turbo'ed gasoline engines are a minute portion of the vehicle fleet. While these cars can be a powerhouse compared to the NA stablemates, they all come at a premium price for acquisition and maintenance which is hard to justify for most of the car buying public today. There tends to be a conflict between the desire to go fast(er) and many folk's checkbooks ... "speed costs money, how fast do you want to go?" is still the byword in this performance market segment.
The day has not yet come to the high country where most of the cars here on the road will require premium fuel to achieve their normal performance.
Here's some more info for flyover country: Many of today's vehicles get better fuel economy at high altitude because their computerized fuel systems DO compensate for altitude and lean the engine out at higher elevations. Performance may be degraded slightly, but fuel economy is better. My 4-cylinder gasoline sedan will get 35-40 mpg in mountain driving at 5,000-10,000 ft., but the fuel economy will drop to 30-35 mpg at elevations less than 2,000 feet. I have full fuel economy records that show it. It runs just fine in high-altitude mountain driving on 85 octane and I certainly do not baby it on the passes. Same with my "beater" 4WD. It's never had anything but 85 octane and it passed 100,000 miles long ago--runs great, uses no oil, and runs the passes just fine.
As to gasoline turbocharged engines, many of the newer ones are, in fact, designed to run on regular unleaded fuel. That includes nearly all of Ford's new EcoBoost engines, as well as some others. Because diesel fuel and gasoline prices are likely to remain inverted--and the price of both is going to continue to escalate over the long term--I suspect that we will see a lot more small-displacement turbocharged gasoline engines that are designed to run on regular gasoline.
The volume if "air" may be the same but the particles are not.
As you go up in altitude the amount of oxygen(particles) goes down.
so the charge is different at sea level that it is at 5000ft or higher.
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Originally Posted by Flyover_Country
The turbo will pressurize intake air to the exact same pressure at any elevation. You get the same amount of air particles in your cylinder at any elevation and you will need the exact same octane at sea level as in the mountains..
ps and I got chastised for posting off topic....roflmao
jazz clearly qualified the use of the lower octane fuel to the vehicles designed to run on it, which is most of what you'll see on the roads around here.
While latest gasoline engine technology has us seeing "10-12:1" and as high as 14:1 direct injection gasoline engines now coming into production ... with all the latest technology to capture the efficiency of a higher compression gasoline engine ....
they are a miniscule portion of the vehicle fleet on the road today, probably less than 1% of the cars on the road.
Many cars made in the past 10-15 years have 10.0:1 compression or better. Compression ratios went up when cars moved from V8s and large-displacement pushrod V6s to smaller-displacement DOHC V6s and four-bangers. I am most familiar with Ford engines so I will use them as an example. Ford started putting 10.0:1 compression 2.0 Zetec fours and 3.0 Duratec V6s in many vehicles starting around 2000. There are many millions of these units out on the roads. The follow-on MZR four-bangers and 3.5/3.7 Duratec V6s generally have an even higher CR.
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As well, turbo'ed gasoline engines are a minute portion of the vehicle fleet. While these cars can be a powerhouse compared to the NA stablemates, they all come at a premium price for acquisition and maintenance which is hard to justify for most of the car buying public today. There tends to be a conflict between the desire to go fast(er) and many folk's checkbooks ... "speed costs money, how fast do you want to go?" is still the byword in this performance market segment.
The reason the turbo engines are here is fuel prices and CAFE mandates. It's not "well, we'll bolt a turbo to our existing engine to make it go faster," it's "we'll bolt a turbo on an engine with two fewer cylinders or 1/2-2/3 the displacement to give it the same power but better low-load mileage."
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Originally Posted by snofarmer
Jazzlover, is spot on.
The volume if "air" may be the same but the particles are not.
As you go up in altitude the amount of oxygen(particles) goes down.
so the charge is different at sea level that it is at 5000ft or higher.
ps and I got chastised for posting off topic....roflmao
The volume of intake air is not the same, the post-turbo pressure is. We are not dealing with a volumetric sensor, turbos deal with a fixed output pressure wastegate. You will get the same charge pressure due to the turbo continuing to work until it hits the blow-off pressure and there will be a nearly identical amount of air molecules in it. The turbo will just have to suck in more air to reach its operating pressure.
Yes And at altitude that air will have fewer oxygen molecules no matter how much you pressurize it,
ps most if not all waste-gates are on the exhaust side not the compressor side.,
and none of the diesels I've worked on had a waest-gate on the compressor side.
A waestgate is used to relieve drive pressure on the exhaust side of the turbo.
Quote:
Originally Posted by Flyover_Country
You will get the same charge pressure due to the turbo continuing to work until it hits the blow-off pressure and there will be a nearly identical amount of air molecules in it. The turbo will just have to suck in more air to reach its operating pressure.
Last edited by snofarmer; 08-24-2012 at 08:23 AM..
Yes And at altitude that air will have fewer oxygen molecules no matter how much you pressurize it
It will have the same number of oxygen molecules unless the temperature is different. The Ideal Gas Law (PV = nRT) explains this.
P = pressure
V = volume
n = number of moles of gas, where one mole = 6.022x10^23 molecules of the substance
R = gas constant, is fixed for any particular gas
T = temperature
The turbo pressure will be constant as the turbo provides X psi of boost regardless of altitude. The volume is constant as well as this is the volume of the cylinder and intake, and does not change with altitude. We are still dealing with air so R does not change, and since we are only talking about altitude, let's say the temperature is the same as well. P*V is thus constant and so is R*T, thus the number of air molecules is the same as well. So we will have the same amount of air molecules present in the cylinder as at sea level and we will need just as much fuel of the same octane as at sea level to have proper combustion.
We can explain why naturally aspirated engines don't need the same octane as well with the ideal gas law. The pressure in the cylinder at the bottom of the stroke is atmospheric pressure, which is lower at 5000 feet than at sea level- not constant like as in a turbo motor. The cylinder volume is again the same, and R and T are again the same. P*V at high altitude is lower than at sea level and with R and T being constant, we have less n, or fewer molecules of air in the cylinder. The cylinder has a fixed compression ratio and so we will also have a lower pressure at TDC than at sea level. The compression ratio to octane/timing maps are determined at sea level and this lower TDC pressure at high altitudes is equal to if you have lower compression pistons in at sea level, and thus you need less octane to have knock-free combustion. You will eventually run into issues where if you go high enough your turbo may not be able to move enough volume of air through it to make full boost, but if WWII reciprocating engine aircraft are any example, it takes a lot of altitude to really thin out the air to need greater volumes out of forced induction mechanisms (they used multi-speed superchargers to overcome this.)
The reason you can run a NA engine operating in those conditions lean is because there are O2 sensors. Combustion is a chemical reaction and fewer molecules of oxygen in the cylinder to combust the gasoline will have the engine run rich. The O2 sensors will sense this and lean out the engine appropriately. However less oxygen + less fuel = engine that runs fine but makes much less power. Turbo engines don't have that issue and are well known to perform considerably better than NA engines at high altitude.
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ps most if not all waste-gates are on the exhaust side not the compressor side.,
and none of the diesels I've worked on had a waest-gate on the compressor side.
A waestgate is used to relieve drive pressure on the exhaust side of the turbo.
It doesn't matter if you are blowing off post-compressor boost air or blowing off exhaust on the exhaust side, you are still regulating the boost pressure. That was my point.
Many cars made in the past 10-15 years have 10.0:1 compression or better. Compression ratios went up when cars moved from V8s and large-displacement pushrod V6s to smaller-displacement DOHC V6s and four-bangers.
And many of those cars have incorporated the technology to allow them to run on regular gasoline. For many cost conscious transportation car buyers, the increased cost of operation per tankful on premium isn't justified over a stablemate car using regular gas.
The reason the turbo engines are here is fuel prices and CAFE mandates. It's not "well, we'll bolt a turbo to our existing engine to make it go faster," it's "we'll bolt a turbo on an engine with two fewer cylinders or 1/2-2/3 the displacement to give it the same power but better low-load mileage."
Having watched the sales of stablemate cars over the last decades where they offered NA and turbo'ed power in essentially the same chassis, I've had a chance to compare that "smaller displacement turbo'ed engine" model vs the NA with a slightly larger displacement. In the hands of an average driver in average circumstances and use, the NA models have delivered equal or better fuel economy ... in the real world of transportation. The turbo'ed models have been targeted to the performance end of the buyer appeal for sales, with the concept being that you can have a smaller displacement engine for fuel economy but have the turbo boost when you need the performance; in the real world of driving, it appears that most of the drivers of these cars utilize the "performance" characteristics to the detriment of possible fuel economy.
So, to bring this discussion back to the issue of fuel price "inversion" and what it does to driving economics, I checked fuel prices in my area today. Diesel fuel is priced 10% higher than gasoline nearly everywhere. In absolute terms, where I fueled today, that translated to 37 cents a gallon cost premium for diesel fuel. So, it follows that diesel would have to get a least 10% better fuel economy to equal the same fuel cost per mile as the gasoline-powered vehicle, assuming the two vehicles had the same longevity, and acquisition and maintenance costs. In my particular case, the diesel pickup that I regularly drive gets 25% better fuel economy than a comparable gas engine-equipped pickup, and often up to 35% better fuel economy. So, the diesel handily wins on fuel cost per mile, even with the fuel price inversion. What about everything else? That's murkier. The diesel commands around a $7,000 acquisition cost premium, which must be amortized over the life of the vehicle. Typically, diesels also cost a little more to maintain, but the engine life before a major overhaul is necessary is usually longer for a diesel engine. In my calculations, taking those things into account, the diesel still comes in ahead in overall operating costs over the life of the vehicle, but not by a whole lot. The "tipper" for me, as I noted earlier, is the diesel's superior performance and fuel economy when working at high altitude. For most people, however, that last point would not be factor, since they don't have to deal with high-altitude driving on a daily basis.
Lastly, the fuel economy of the latest model diesel engines is pretty amazing. I calculated the fuel economy per pound of empty vehicle weight for the diesel pickup that I drive and factored that to the empty weight of my 4-cylinder economy sedan. If the sedan was to achieve the same fuel economy per pound of empty vehicle weight of the pickup, it would have to get 60 mpg on the highway. About the best it can get is 40 mpg, 33% less fuel economy per pound of vehicle weight. By the way, a diesel version of a very similar economy car to mine manufactured by the same company and sold in Europe does achieve around 50-55 mpg (US measure) with better performance than my US gas engine version.
So, to bring this discussion back to the issue of fuel price "inversion" and what it does to driving economics, I checked fuel prices in my area today. Diesel fuel is priced 10% higher than gasoline nearly everywhere. In absolute terms, where I fueled today, that translated to 37 cents a gallon cost premium for diesel fuel. So, it follows that diesel would have to get a least 10% better fuel economy to equal the same fuel cost per mile as the gasoline-powered vehicle, assuming the two vehicles had the same longevity, and acquisition and maintenance costs. In my particular case, the diesel pickup that I regularly drive gets 25% better fuel economy than a comparable gas engine-equipped pickup, and often up to 35% better fuel economy. So, the diesel handily wins on fuel cost per mile, even with the fuel price inversion. What about everything else? That's murkier. The diesel commands around a $7,000 acquisition cost premium, which must be amortized over the life of the vehicle. Typically, diesels also cost a little more to maintain, but the engine life before a major overhaul is necessary is usually longer for a diesel engine. In my calculations, taking those things into account, the diesel still comes in ahead in overall operating costs over the life of the vehicle, but not by a whole lot. The "tipper" for me, as I noted earlier, is the diesel's superior performance and fuel economy when working at high altitude. For most people, however, that last point would not be factor, since they don't have to deal with high-altitude driving on a daily basis.
Lastly, the fuel economy of the latest model diesel engines is pretty amazing. I calculated the fuel economy per pound of empty vehicle weight for the diesel pickup that I drive and factored that to the empty weight of my 4-cylinder economy sedan. If the sedan was to achieve the same fuel economy per pound of empty vehicle weight of the pickup, it would have to get 60 mpg on the highway. About the best it can get is 40 mpg, 33% less fuel economy per pound of vehicle weight. By the way, a diesel version of a very similar economy car to mine manufactured by the same company and sold in Europe does achieve around 50-55 mpg (US measure) with better performance than my US gas engine version.
But here again, we've digressed into the world of trucks and not the real world numbers of CARS in the marketplace.
Unfortunately, these numbers simply don't apply to the products available in the CAR marketplace here in the USA. High numbers in the diesel car game today are 50+/- mpg achieved with a $7,000 price differential to the gasoline stablemate that can run into the mid-to-high 30's. My local fuel supply was at $3.39 regular gas vs $4.19 #2 diesel .... or .0971 per mile vs .0838 per mile. At less than 2 cents per mile fuel price advantage, it's lot of miles ahead to make a payback with the diesel.
Diesel cars made economic sense when the cost of diesel fuel was far less than gasoline and there was a huge disparity in the economic service life of the engines. But that situation has not been the case for a long time. That's why my remaining diesel car is parked in favor of my far more economical gas cars and I keep my diesel pick-up trucks exclusively for the days when I need to haul a load for the farm.
When diesel cars can again achieve a significant cost advantage over gasssers, I'll drive them again. But it's not happening now.
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