In reply to SVreX:
Need to scale that example at least up to an EU + North America thing.
The EU has been favoring diesel for some time now- to try to meet the Kyoto agreements. To the detriment of their air quality- which you can see the reaction in the Euro V and VI emissions rules. The tax and emissions rules are both much toward the diesels.
In reply to alfadriver:
Well, right. I wasn't trying to account for all of the factors.
My point was:
1- Diesel was NOT always more expensive than gas (it is a fairly recent phenomenon), and
2- The difference is NOT because of the tax differences.
There are many factors. The primary one is the imbalance between manufacturing capability and demand (which accelerates more when you include the EU and other countries).
Another thing about gas vs. diesel, given the direction engine development has been going, give it another 10 years and the gas engines will probably be able to run both without much modification.
rcutclif wrote: Personally I believe the fuel usage calculated by the car's computer is MUCH more accurate than the meter on most pumps, and let's face it, if you don't use the car's computer number, you have to use data that comes only from your pump's meter at the gas station.
The cars mpg calculations are as accurate as it's speedometer sensor reading. The pump is as accurate as your local department of weights and measures guy is moral. 4.8 vs 5 is a 4% difference, about how far off most speedos are.
In reply to Kenny_McCormic:
I don't think cars generally use the speedometer to calculate fuel economy.
It's measured injector pulses per miles driven (which can be derived in several fashions).
Kenny_McCormic wrote: Another thing about gas vs. diesel, given the direction engine development has been going, give it another 10 years and the gas engines will probably be able to run both without much modification.rcutclif wrote: Personally I believe the fuel usage calculated by the car's computer is MUCH more accurate than the meter on most pumps, and let's face it, if you don't use the car's computer number, you have to use data that comes only from your pump's meter at the gas station.The cars mpg calculations are as accurate as it's speedometer sensor reading. The pump is as accurate as your local department of weights and measures guy is moral. 4.8 vs 5 is a 4% difference, about how far off most speedos are.
But you still use the same error in your car's speedo when you use the pump's measurement. I guess the point is there are two ways to measure the G in MPG, but usually both use the same thing to measure M.
I guess you can measure your car's speedo against a GPS or highway markers, but how many people take that error into calculation when fueling up and calculating MPG?
In reply to SVreX:
How do you derive miles driven without having to assume a tire diameter?
Yup, tire diameter, or GPS position if you're a baller...
Adrian_Thompson wrote: Aerodynamics, smooth driving, don't use cruise control. Try and avoid accelerating or braking. I did get 30mpg on some occasions driving 80mph in our Mustang convertible, but the only way to do that was on cruise control. As soon as you are changing speed you're screwed.
If its completely flat, cruise control....if there are hills, use your foot. I've always gotten worse with cruise on.
SVreX wrote: Prior to 2004, gasoline generally exceeded the cost of diesel at the pumps in the US. Diesel was cheaper.
I don't recall that.
I used to have a picture from that timeframe with 87 at $1.49 and #2 at $2.99.
I’ve never understood the fascination and admiration for diesel on this board. For commercial use, or fleets of truck where they get driven 40-50-60K miles per year yes no problem. Again if you want a tow vehicle like a truck, Excursion or Touareg for towing your race car, boat, horse trailer no problem, it can make sense. But to me they don’t make sense for private vehicles. The majority of the automotive public don’t consider a 90’s Honda with 150K miles as barely run in. Nor do they think of a 30 year old Rabbit diesel with 300k on the clock as just getting into its stride. Most people drive 10-20K miles a year and keep a car 1-7 years. It’s really hard to make any kind of case for a diesel under those conditions. Fuel is simply too cheap over here for the economics to make sense. It works in Europe where fuel is actually expensive, but here’s it’s always been cheap. Fuel costs in Europe are easily 2.5 – 4 times what they are here. The Current Golf TDI gets 20% better fuel econ than the best gas offering yet costs 22% more. If you’re driving the average 15K miles a year your currently only saving $200 a year in fuel for the Diesel, that’s going to take 30 years to even break even. OK, let’s say you do a lot of miles and gas has gone up to $3.5 and diesel is $4.5. Then you make out better on the miles, but lose out on the extra cost so it’s still only a few hundred bucks a year. Then add in I’ve never driven a fun diesel car. All that torque is great in SUV’s and trucks, but not in cars. I grew up in the UK and visit relatively frequently so have driven dozens of diesel cars over thousands and thousands of miles. Once you get of the first few mins novelty of all that low end torque, you soon get pissed off at the low red line and lack of top end power. The best diesels are at best on par with the weakest gas engines for fun to drive.
They work in Europe because of the incredibly high gas prices, but even there they are falling out of favor. The Mayor of Paris is calling for a ban on diesels by 2020 due to the pollution, they are still a long way behind gas engines overall.
Trucks, semi’s, big payloads, towing trailers, yup, all great uses for diesels, but passenger cars, keep them a long long way away from me.
In reply to Adrian_Thompson:
I can honestly agree that I've not liked any of the diesel cars I've rented in Europe. From a really really slow Opel to a very very boring Alfa GT. The gas 155 and 156 we've had were really fun.
But I'm just one person. If you really want a diesel- get the ones you can and love them.
In reply to Adrian_Thompson:
Diesel car marketing works in mysterious ways....
But I totally agree. Side note: my focus svt and my 84 f250 diesel both make 170HP. One is fun to drive the other is good at towing. Neither does both (at all).
My simplistic understanding of ideal highway MPG (steady state, not necessarily the crazy EPA test) is that best fuel economy occurs when you can gear the car to require a torque output at an rpm that hits the best Brake Specific Fuel Consumption (BSFC) possible. A BFSC chart (below) shows the "fuel per output" (grams of fuel per kw of output, typically)required for combinations of torque and rpm.
This is a very typical-looking BSFC map. It seems obvious that, up to a point, higher torque is better (gear it taller) and lower rpm is better (gear it taller). Best highway fuel economy is going to happen if you can get the engine operating in that 250 g/kw.hr zone at highway speeds.
curtis73 wrote: I'll start with the things that can change easily instead of diving right into building an engine for MPG. [snip] - Throttle position. I don't like discussing throttle position because it eventually turns to "pumping losses." It makes me want to punch people in the boob.
Why? If you talk to an OEM calibrator, he's going to say it a lot. What limited things I know came from a few such people on another board.
For any given combination, there are many factors that come into play. Throttle position is not synonymous with throttle flow. The difference between 0 throttle and 10% throttle is on the order of 500% change in airflow, but the difference between 90-100% throttle is on the order of 10%.
In fact, 90 and 100% may be functionally identical (there's no pressure loss across the plate), which is why you sometimes see wide open report less than 100% at the TPS.
""
Generally, the least throttle position will net the best MPG, but not always.
That's just false. I used to own a Chevy Volt, which has the advantage of having no fixed relationship between engine speed and vehicle speed. Operating in hybrid mode, the throttle plate was almost always wide open, using the generator to load the engine down to an ideal Brake Specific Fuel Consumption point. It did most of its running at 1700, 2100, and 2800rpm - all with the throttle wide open, all very good BSFC points, with 2800 being the best but not so great on the NVH scale.
- RPMs. Any talk about lower RPMs being better is horse poop. Take a look at the torque output of your engine. If you torque peaks at 4500 rpms, using a really low highway RPM is pointless for two reasons; 1) you will be operating where the engine makes pathetic torque and therefore require greater throttle openings to make enough torque to overcome moving resistance. 2) if your torque peaks at 4500 rpms, chances are that your cam timing events (specifically overlap) are favoring higher RPM cylinder pressures. Operating at RPMS that are too low will cause far greater intake reversion, intake dilution, and MPGS will suffer. The *proper* RPM is where you will get the best MPG, not necessarily lower. My current build will be spinning 2400 rpms on the highway, and I can almost guarantee that I could gain some MPGs by spinning it at 2600 because of the large amount of overlap. Spinning it faster (in my case) would increase VE by trapping more intake charge which increases pressure which increases the amount of BTUs released by the fuel as combustion pressure.
It certainly is not poop. Why does a Corvette come with a 7 speed transmission? To reduce frictional losses and let them load the engine to the moon for a large throttle opening. Your point about valve overlap probably has some merit in a world without variable valve timing but what modern engine has fixed valve timing? The Saturn I posted above seems to make peak torque around 5000rpm but that's way off the reservation for ideal fuel economy, especially since gearing it to hit the higher rpm would mean lower torque requirements.
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curtis73 wrote:Rupert wrote: In reply to curtis73: I've read more than one or two treatises that say the best throttle position with conventional carbs. is wide open while pulling the highest gear possible. You're operating as close to engine stall speed as possible. This results in a higher air to fuel ratio since the suction which draws the fuel is hugely reduced. But watch out for compression knock!! Pin holes in pistons can get expensive.While that is true for thermal efficiency (getting the highest amount of BTUs out of the fuel) it is still moving WAY more fuel than small throttle openings. Thermal efficiency is high, but MPGs drop to the single digits.
Sorry, you are wrong on this one. Fuel mileage often doubles at wide open throttle. Converting fuel into BTUS is what makes the torque, that runs the car. And getting a the highest BTUs out of the fuel is the gold standard of achieving fuel efficiency.
In reply to Adrian_Thompson:
I never understood it either. People complain about automatics, need a manual trans, gotta have a manual trans, then they worship engines that don't really need to be shifted in the first place.
Call me a party pooper, but I love my 00 Golf tdi and can't complain about it still getting 50 mpg even though I drive it like I stole it... everyday.
mfennell wrote: A BFSC chart (below) shows the "fuel per output" (grams of fuel per kw of output, typically)required for combinations of torque and rpm.
A BSFC chart! Awesome. Time to get some math up in this business.
Now, let's look closely at this. Let's say you want to gear the car so that you are pulling 2000 rpm at 124 ft-lb of torque. That is about 50hp. How fast are you going to be travelling to require 50hp? A common thing I remember for modern cars is roughly 25hp for 60mph. If power required goes up as the square of speed, then doubling the power requirement should be the speed times the square root of 2. So when you get pulled over for driving your Saturn at 85mph, you can just politely point out that you were trying to run your engine where it was most efficient.
Of course, if you were driving at the speed limit, while the engine won't be running at peak BSFC, it would also only need half the power output. The BSFC isn't doubling under low load.
Now, with respect to rotaries, which CAN see the BSFC double under low load and/or low RPM, sometimes you get the best fuel economy by driving a lot faster.
Why does a Corvette come with a 7 speed transmission? To reduce frictional losses and let them load the engine to the moon for a large throttle opening.
Funny you should mention that. They had a hard time going away from the 4 speed automatic because the engine's efficiency islands were so broad that more gears resulted in poorer fuel economy, because the added internal drag in the transmission was not enough to offset the gains from more precise gearing.
Re- BSFC charts...
Couple of things. First of all, you know if you do the math, it's actually a dimensionless unit. grams of fuel/hr = power, and you are dividing by power. Some nice unit conversions + replacing the fuel with the energy of the fuel, and you end up with 1/thermal efficiency.
The other thing- as knurled pointed out- cars use power to move at a speed (power= force*velocity)- so while it's common to plot BSFC vs. some kind of torque (many times it's plotted against mean effective pressure)- it's actually a lot more interesting when you plot vs. power. Which is where you find the errors in state that I'm pretty sure Curtis is getting at. So it takes 25hp to go 60mph flat. It takes a little less going down hill, and a little more to go up hill. So to maintain 60mph + some hills + some speed errors- it's a power island you are looking at. When you plot nice and round islands in torque domain into power, the shape changes a lot- to the point where you see lower speed isn't always better to maintain your car in the real world. Which changes the gear ratios or perhaps what's the best desired gear to be in for an engine and speed.
One of the things that makes engine people think... some have a tough time translating their great dyno data to cars.
Knurled wrote:SVreX wrote: Prior to 2004, gasoline generally exceeded the cost of diesel at the pumps in the US. Diesel was cheaper.I don't recall that. I used to have a picture from that timeframe with 87 at $1.49 and #2 at $2.99.
Your recollections are incorrect.
I see now that I made an error in my math. The chart is calibrated in fig-newtons instead of stone-furlongs. So the figures will be a little bit more realistic, but the point still stands that the engine and transmission and chassis end up having to be a system, with the engine engineered to get the best BSFC at X speed, with the chassis determining how much power it takes to do it. And since we're studying to pass a test (the CAFE), as soon as you go much outside of those bounds, fuel economy probably gets worse and "your mileage may vary".
The Volvo's trip computer is neet to play with. It has total and instant (5 second lag-time) economy readouts. The total is pretty accurate compared to actual distance vs. pump shutoff to pump shutoff, so I am inclined to trust the instant. Fuel economy going downhill definitely does not make up for the lack of economy going uphill. When all is flat, I've seen 36mpg, but when driving on hilly stuff it nosedives to 33 average. Sure it does 70-99mpg downhill but the 20 uphill eats that up fast.
My RX-7, incidentally, probably couldn't break 36mpg going downhill, just because of idiosyncrasies with Megasquirt. MadScientistMatt may be able to chime in but I can't get duty cycle to drop below 9% when coasting in-gear. 9% duty cycle with my injectors is 23 hp worth of fuel :(
Knurled wrote: How fast are you going to be travelling to require 50hp? A common thing I remember for modern cars is roughly 25hp for 60mph.
FWIW, that Saturn takes 10.7kw (at the wheels) to move @ 60mph. 50 crank hp (say ~42whp), would be around 93mph... so you're pretty close.
Kenny_McCormic wrote: In reply to SVreX: How do you derive miles driven without having to assume a tire diameter?
How did you get from speedometer to tire diameter? I didn't say anything about tire diameter.
Tire diameter can be presumed without a speedometer. Known tire diameter and gear ratio can give a very precise distance based on driven gear rotations.
GPS is another way.
There are probably others, but I am not an automotive engineer.
Knurled wrote: The Volvo's trip computer is neet to play with. It has total and instant (5 second lag-time) economy readouts. The total is pretty accurate compared to actual distance vs. pump shutoff to pump shutoff, so I am inclined to trust the instant. Fuel economy going downhill definitely does not make up for the lack of economy going uphill. When all is flat, I've seen 36mpg, but when driving on hilly stuff it nosedives to 33 average. Sure it does 70-99mpg downhill but the 20 uphill eats that up fast.
Interesting. I've had that experience with other cars as well. My Saturns have always gotten good economy in the mountains through WV - probably a combination of being right in that sweet spot at 80mph (near WOT), and DFCO going back downhill.
The Prius I have will get very good economy in the mountains compared to flat interstate travel. Pretty much always in the sweet spot (which it really doesn't have), and massive recovery/coasting downhill.
SVreX wrote: Your recollections are incorrect. 2006 OLR Research Report
I am sure regionality plays a big part too. Why, I was driving somewhere last year and I saw diesel 10c cheaper than 87, while it was 50c more expensive here.
Another thing is that the price fluctuates a heck of a lot less. The station by me has had diesel at $3.89 for the past... month maybe? Gasoline is now $2.36 for 87, it was $3.09 or something a month ago.
In reply to Knurled:
Did you read the article?
Of course there are regional variations. But that doesn't change the fact that diesel was consistently cheaper than gasoline for many decades in the US.
And those wild fluctuations were largely non-existent as well.
Ask any trucker over the age of 50.
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