Gas versus diesel


Little article about how effecient diesel engines are, but how quickly gasoline engines are catching up.

When it comes to traversing great distances at highway speeds, the diesel engine’s higher compression ratios and lean-burn combustion provide an efficiency that no gas engine can currently match?at least not without a major assist from an expensive hybrid system. Over the diesel’s operating range, the average thermodynamic efficiency?how much work the engine produces from the fuel?is in the mid 30 percent range, at least 15 percent better than a gas engine. Not even close, right?

The reality is that this lead is shrinking. As emissions regulations stiffen, diesels are slowly losing their edge; the same pricey after-treatment systems that scrub diesel exhausts also happen to crimp efficiency. Meanwhile, gas engines continue to improve.

Agree that with direct injection and higher compression ratios, the gas engine can approach the diesel in effiency without the elaborate exhaust after treatment.

Expect diesels to remain the workhorses of the truck, marine, power genaration, and industrial engines. The bottom of the barrel fuels can only be used in large diesels or as a furnace fuel.

Hybrids will be an increasingly common part of the equation, and hopefully the cost will come down to where they’re comparable in price to non-hybrids.

Interestingly, the EPA has begun to address diesel emissions for big rigs, and I’ll guess that as this process unfolds hybrid diesel rigs will begin on an evolutionary path. 80,000 pounds traveling at 70 mph would give whole new meaning to “regenerative braking”!

This is one of those dumb articles that ends up contradicting itself in the last paragraph:

Quote: As GM’s Grebe points out, diesel fuel contains about 14 percent more energy by volume than gasoline. This gives compression-ignition engines a significant edge in fuel economy, as opposed to thermal efficiency.

Yeah, gasoline engines are improving, but so are diesels. Maybe a better title for the article would have been “Gasoline engines are improving, and so are diesels.” OK…big surprise.

A gasoline engine actually can match or nearly match the thermodynamic efficiency of a diesel when it is running in its sweet spot. That’s why on the new Volt, the engine is either off or running at nearly full throttle, or so I hear.

It is claimed that about 17% of the gas a car burns in stop and go traffic is idle fuel consumption. This is where diesels really have tha advantage. A diesel’s idle fuel consumption is typically 1/3 to 1/4 that of a comparable gasoline engine.
It is idle and low power output fuel consumption where the real gains are being made. A Toyota Yaris burns about .16 gallons per hour idleing where an equivilent older low tech engine would have consumed around .25-.3 gph. This is due largly to the variable valve timing, the engine switches to a super late intake valve closing timing allowing the pistons to push the extra charge back into the intake manifold instead of fighting the high vacuum of a closed throttle. This results in a less parasitic intake stroke.
Also, turbocharging allows a two liter engine to make four liter power while still having a two liter engine’s idle fuel consumption.

Diesel is too valuable to be use for personal transportation. Even in hybrid use, little beats the old push rod/carb gas engine for efficiency if it just has to operate over a limited rpm range in a series hybrid like the Volt. Why bother…gas is cheaper, gas motors are cheaper and lighter to build, all a big advantage and non competitive with heavy haul, industrial and marine applications where diesel rules. I would look for a comeback of the gas two stroke where Bombardier has shown proven technology in clean marine. Just a little jump to cars…who wants a diesel weed wacker ?

While the turbo does add that, it adds a whole new set of possible problems for some(probably most) drivers. The cooling down of a turbo is still something that’ll need to be addressed by most car makers.
While my manual states to let the engine idle for 30 seconds or more after a highway run, how many people are gonna get off the highway after they’ve cruised at 80mph+ for a few hours, pull in the gas station and shut the engine off right away. That should coke the oil in no time

The bottom of the barrel fuels can only be used in large diesels or as a furnace fuel.

Actually, oil refinerys have been catylist-cracking the heavier factions into gasoline since the days of WWII. Before then, oil companies practically gave kerosene away because it was a by-product of gasoline refining. A lot of old farm tractors had the intake manifold going through the exhaust manifold so the engine could run on cheaply available kerosene once the engine was warm.
Before the gasoline engine was invented, it was gasoline that was practically given away because it was a by product of fuel oil and kerosene refining.

BLE, there are bottoms and bottoms. Everyone knows that it is economical to crack the middle of the barrel and somewhat lower fractions, into gasoline.

The very bottom is ashphalt, other semi-liquid fractions, and a very heavy residual often called “bunker C”, which is a good fuel for very large diesels. To create a light motor fuel out of this is technically possible, but prohibitively expensive! I worked on a project on “Supercritical Extraction” which tried to make a liquid fuel out of the heavy residual of oilsands refining. It simply was not economical.

Any book on refining will show what the various fractions are best suited to. A good modern refinery has the flexibility to produce a complete range of products out of a variable feed of crude oil.

Quote: As GM’s Grebe points out, diesel fuel contains about 14 percent more energy by volume than gasoline. This gives compression-ignition engines a significant edge in fuel economy, as opposed to thermal efficiency.

Well, diesel has 14% more energy per gallon…and weighs close to 14% more per gallon. Thus, comparing a gallon of diesel to a gallon of gas is like comparing apples to…14% bigger apples.

Whether that constitutes a fuel ECONOMY edge seems determinant on the price of a gallon of Diesel to a gallon of gasoline: does a higher per-gallon price totally eat up the benefit of higher BTU/gallon?

I’ve worked gas and diesel on heavy truck…gas is a bad choice for heavy truck. Perhaps that’s why no one makes large gas engines anymore. MPG is misleading. There is more energy in a gallon of diesel(more Carbon double bonds) than there is in a gallon of gasoline. Ideally, we shuld be buying gasoline in kW-h or BTUs or something. A warm gallon of gas(expansion) has less energy than a cold one and don’t dump a bunch of nitrogen in and claim I’m getting something special…all I’m getting is less energy (nitrogen doesn’t burn) but it does take up space. Buy gas cold, unoxygenated and undistrurbed for a few days…hahah.

One of the advantages that diesel has over gasoline that will never be made up for is safety. For gasoline to achieve it’s performance, it’s volatility must be dealt with. Diesel and it’s “sister fuels” fuel oil are so safe, they can be stored in our homes, fuel aux. and main marine engines and provide safer refueling for jet engines.

Regardless, diesel is too valuable a heavy haul and special usage commodity to ever be left to the realm passenger car transportation on a large scale. It would “tick me off” to no end if it were ever considered for professional racing on a large scale or commonly used for a teens “joy ride”.

Gasoline is better at keeping the rear of the car body clean, but diesel fuel is free with the right equipment after dark. Choices, choices.