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We have the diesel model, just one week old. So far we have had two breakdowns, battery totally flat both times. When we first picked up the car, the dealer advised turning the ignition on so that all lights (airbag, fuel etc) were showing, waiting until the lights went off, then turning over the engine. Today we were stuck on a ferry and had to stay on board for two more crossings whilst the crew jump started our totally flat battery - my husband had turned the ignition on for a whole 3 minutes before turning over the engine! Hoping to get to the bottom of what is a really ridiculous problem to have in a brand new car.
Were told by fellow passengers who have diesel motors that the battery needs to be much stronger than the one sold with a new Volvo. Will keep you posted on what dealer says! :confuse:
Other than that very happy with the car, fantastic ride, and given that we have the 2.4D 126 horsepower model, it still has plenty of power for suburban driving.
I have thought about diesels for a long time but have always just bought gasoline engined vehicles because they are cheaper and have always had lower tailpipe emissions.
The biggest single factor in the higher mpg obtained with diesels is that diesel fuel is about 17% denser than gasoline. A liter, or a gallon, of diesel fuel weighs 17% more than the same volume of gasoline, so it contains 17% more fuel, and carbon. It is the weight of fuel consumed and not the volume of fuel consumed which determines the level of tailpipe emissions of carbon dioxide. On a weight basis diesel fuel and gasoline release almost exactly the same amount of heat energy. The diesel cycle, however, converts somewhat more of this heat into mechanical power.
Then there are the problems of NOx and fine particulates. Diesels are somewhat more efficient than spark ignition petrol engines, but one way they achieve this is by pumping a lot more air through the cylinders per unit weight of fuel. This causes more NOx than spark ignition engines in which the air is of necessity "throttled" to keep the fuel-air ratio in the range where a spark can ignite it.
Modern diesels from VW and Mercedes are much, much cleaner, but this is being obtained by the use of special emissions controls systems which add to the cost and slightly reduce the efficiency.
Another way that diesels improve their apparent efficiency over gasoline engines is that the diesel option is usually of lower horsepower than the gasoline option. A gasoline engine of the same max power rating as the diesel would usually have higher efficiency than the higher power one offered.
The true efficiency advantage of the diesel may not be enough to justify the higher lifecycle cost of purchase and maintenance. I say "may be" because I don't know enough to have a professional opinion on this question.
But my nonprofessional opinion is that when comparing diesel mpg to that of a gasoline engine you ought to divide the diesel mpg by 1.17 for a true comparison based weight of fuel consumed. (Or multiply the L/100km value of the diesel by 1.17.) I know fuel is sold by volume, but if diesel and gasoline were sold on a equal heat of combustion basis, then a litre or a gallon of diesel would cost 1.17 times the same volume of gasoline. Someday this may be the case.
In chemical reactions (like combustion) there is a law of conservation of mass, but there is no law of conservation of volume.
You bring up fact that the density of fuel changes with temperature. It does so because the volume changes with temperature. The mass of a given amount of fuel does not change with temperature.
The output torque and power of an engine/tranny combination is what is important, not the maximum torque of the engine itself. The output torque from the tranny can be increased by changing the gearing. But the maximum rated power (HP rating) of the engine/tranny combo cannot be changed--the engine can only develop so much power. A transmission can multiply torque but cannot multiply power.
Diesels have some advantages over gasoline engines
(1) The diesel cycle is somewhat more thermodynamically efficient on a mass basis, the only true basis for comparison.
(2) Diesel fuel is less flammable than gasoline
(3) The diesel exhaust stream is significantly lower in temperature than that of a gasoline engine so turbochargers in diesel engines have much less heat stress.
I like diesels, but I don't think the sellers of diesels should get away with misrepresenting the extent of the efficiency advange of diesels over gasoline engines.
PS Another reason for the superior efficiency of current diesels over current gasoline engines is that the diesels are direct injection (i.e. fuel sprayed directly into the cylinders) whereas nearly all gas engines use less efficient port-injection. When more gas engines use direct injection the diference between gas and diesel will be even less.
Modern very high pressure "common-rail" fuel systems with piezeoelectric injection has really improved the combustion process in compression ignition engines. The fuel is evidently injected in up to 5 separate injections in each power stroke. This smooths out the energy release and improves efficiency.
I am no scientist but I believe your comments about measuring weight of fuel makes a lot of sense. I am confused by your comment that the torque of an engine (stand alone) is not relevant but rather the engine/tranny combination. If that were true, why do manufacturers publish the torque of an engine and not the engine/tranny. Furthermore, if horse power was the only relevant measurement of power, why are trucks that are used for towing, not using high revving high horse power engines. I believe that they go for large displacement, high torque engines regardless of the gearing. I have no scientific proof but something tells me that the torque of an engine is still relevant.
At any given rpm the relationship between power and torque developed by an engine is Power in hp = torque in lb-ft x rpm / 5252, in metric units Power in kw = torque in Nm x rpm / 9549. The whole torque curve is extremely useful information about an engine, but its main use is for the purpose of selecting the transmission gearing. This is something that the manufacturer does--if it's done right then the vehicle performs well whether its powered by a petrol or a diesel engine.
For the V70 compare the engine specs for the two Volvo 2.4L diesels (120 kw or 161 hp and 136 kw or 182 hp) and the Volvo 2.4L normally aspirated 168 hp gasoline engine (125 kw petrol), and ask yourself whether the much greater cost of the diesels would be worth it to you. Go to http://www.volvocars.co.uk/models/v70/techSpec.htm and compare the two diesels with the 125 kw (168 hp) petrol engine. Note that the mpg values are mpgUK. Divide mpgUK (mi/Imperial Gallon) by 1.2 to get mpgUS. From the acceleration (0 - 62 mph) youcan see that it is power not torque which determines the capability of the vehicle under ordinary circumstances.
I believe that the diesels are better designed and built engines than the petrol one, and they use about 13 % less fuel per unit distance (fuel measured by weight, the true basis of amount of fuel, diesel fuel is about 17 % denser than gasoline) than the petrol engine, and produce about 19% less CO2 emissions per unit distance ( diesel 179 g/km vs 220 g/km for the petrol engine). But the diesels cost a whole lot more in initial outlay and maybe in maintenance costs over the life of the vehicle.
Large diesels in commercial trucks are designed to operate at a very high fraction of maximum rated power and to be very fuel efficient with a very expensive transmission with many gears. In my opinion, the average private vehicle owner doesn't make strong enough demands on their vehicle to justify the extra cost of a Volvo diesel. But if you put a lot of miles on the car, drive at very high speed or tow a heavy trailer then the diesel might be cost effective.
Or maybe you just want the diesel and money is no object.
Amazingly, the 182 hp 2.4D diesel XC90 is cheaper than either of the gasoline engines! This would be the engine for me, if it were available in the US. But it must not meet the US emissions requirements, it probably fails for particulate matter and for NOx. If emissions controls were put on it, the cost would be increased and the fuel economy would drop, but how much I wouldn't know.
But the diesel is considerably more fuel efficient even than the 3.2L I6 (which is what my wife has in her 2007 XC90). The reason why the diesel is more efficient is that diesels are more efficient than gas engines, and this diesel is a smaller engine and so is being used higher in its operating curve. Internal combustion engines get their highest efficiency, the true measure of which is called brake specific horsepower BSHP, when operated high in the performance curve.
Take an example of an engine powered water pump which is required to steadily pump an amount of water requiring a constant 40 hp output of the engine. This job will be more efficiently done with an approximately 50 hp engine than with a 100 hp engine. This is a well known and accepted fact.
To push a moderately loaded XC90 down a highway at 65 mph takes maybe 40 hp (just a guess on my part). In general this would be accomplished more efficiently by a smaller engine than by a larger one. It's just that a small engine will not have the extra power needed to accelerate briskly to enter a freeway safely, to pass, or to climb steep grades at high speed.
A 182 hp engine would be plenty for me.
The true way to compare the efficiency of the engines is with the CO2 emissions
1. Diesel 136 kw (185 PS, 182 hp) 2.4L D 6M ..............219 g/km
2. Diesel 136 kw (185 PS, 182 hp) 2.4L D 6A ..............239 g/km, so 9.1% more fuel per unit distance with the 6 spd auto tranny
3. Gasoline 175 kw (238 PS, 235 hp) 3.2L I6 6A...............281 g/km, so 17.6% more fuel per unit distance than the diesel with auto and 28% more fuel than the diesel with 6M
4. Gasoline 232 kw (315 PS, 311 hp) 4.4L V8 6A ............322 g/km, so 14.6% more fuel per unit distance than the I6, 35% more fuel than the diesel with 6A and 47% more fuel than the diesel with 6M tranny.
In my opinion 126 hp should be plenty of hp for a V70 for restrained driving. It should take only about 20 to 25 hp to push a V70 down the highway at 65 mph.
My 168 hp 2.4L gasoline V70 5A got 34 to 35 mpg at 65 mph on a level interstate highway in the summer on two separate 350 mile legs (700 mi total).
Using the formula for gasoline engines
hp = (speed mph/fuel use in mpg) x 12hp/galUS/hr = (65 mph)/(34 mpg) x 12 = 23 hp.
The diesel powered V70 would require the same hp to go the same speed because it has the same aerodynamics, tires, and tranny efficiency, but the constant in the same type formula for a diesel engine wwill be different because diesel fuel has about 15% more energy per gal than gasoline and the efficiency of the diesel engine is higher than that of a gas engine. A rough estimate of the constant for a diesel might be 16 hp/galUS/hr.
So say that you get 45 mpgUS when driving the 126 hp diesel V70 at a constant 65 mph. The instantaneous power would be given by
hp = (speed mpg)/(fuel use in mpg) x 16 hp/galUS/hr
hp = 65/45 x 16 = 23 hp.
You could figure out a similiar formula using entirely metric units: speed in km/hr, fuel use in L/100km and power in kilowatts. The difference would be that it would be all multiplication because the European fuel use value is the reciprocal of the American mpg expression of fuel use .
So the formula would be
Power in kw = (speed in km/hr)x(fuel use in L/100km)x C, a constant. There would be one constant for gasoline engines and a different one (~33% larger) for diesels.
Power in kW = (speed in km/hr)(fuel use in L/100km)(Constant)
For a gasoline engine the constant is about 0.0236 kW/L/hr, and for a diesel engine the power is about 0.0315 kW/L/hr. (If you'd rather deal with whole numbers then instead of multiplying by the constant you'd divide by 42 for the gas engine and divide by 32 for the diesel.)
1. A 2.4L 125 kW (168 hp) gasoline V70 is traveling a constant 105 km/hr and the computer (or calculations) shows the fuel use is 6.9 L/100km. What power is the engine developing?
Power = (105)(6.9)(0.0236) = 17 kW, or
Power = (105)(6.9)/42 = 17 kW
(Note 17 kW = 17 kW x 1.34hp/kW = 23 hp)
2. A 2.4L diesel V70 is traveling a constant 105 km/hr and the computer (or pump calculations) shows the fuel use is 5.2 L/100km. What power is the engine developing?
Power = (105)(5.2)(0.0315) = 17 kw, or
Power = (105)(5.2)/32 = 17 kW
in USA there are not so many people that had the experience of driving a modern diesel. The massive amount of torque at very low rpm makes for quick from stop acceleration and excellent in gear acceleration. The last one makes for a relaxing drive, as quite often there is no need to downshift with passing.
0-60 indeed has more to do with the engine power, however, on the street nobody-other than few lunatics- are driving '0-60 like' aka dropping the clutch at high rpm at every stop and shifting gears at redline.
I'll say that the majority keeps their engines below 3500 rpm in usual traffic and in that area range the diesels are faster that the similar horsepower gasser.
so far, the only car diesel in USA is the Mercedes 320 cdi. It has 210hp, 400 lbs of torque [more than the 55o v8!] and goes to 60 in 6.6s. the 350-gasser is just $ 1000 less and goes to 60 in 6.5. The mileage, though, is quite different:35/26 for the diesel vs 26/19.
in Europe, even where the diesel is not much cheaper than gasoline, diesel vehicles outsell the gassers...
The dealer said it probably was due to conversion between Europe and US. Called Volvo north America and was told the following: "The trip computer may show more mileage than the odometer if the trip computer is not reset after testing from the factory during production. This is not an indication of a fault with the vehicle. The trip computer and odometer should be reset before the vehicle is delivered.
The trip computer functions independently of the vehicle's main computer and can be reset to zero at any time."
Is this true or convincing? Did anybody else have similar experience? Thanks!
Another question I have is, if somebody turns the odometer backward, will the trip computer follow......