Non hybrids get over 100 MPG now - Page 2 - Fuelly Forums

Android Users - Coming Soon! - Migrating from aCar 4.8 to 5.0

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Old 01-28-2014, 11:11 AM   #11
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It's not really slow though, on paper maybe, but the low down torque and the fact that it has a turbo make diesels feel far quicker than they are. Not all diesels are slow anyway, take a look at the 3.0 litre diesels in Audi and BMW cars, the M6 does 56 MPG and has a 0-60 time of 5.5 seconds.

Its a shame America is getting left behind in many respects, they put man on the moon and yet it's quite outdated on many things, automotive design, gun laws, national health service etc etc
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Old 01-28-2014, 12:50 PM   #12
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Neither are hybrids for the same reasons.

We are just starting to get the Audis and BMWs. It will just take time for perceptions to change. The real hurdle will be the price difference between the fuels. In many areas, diesel is more than premium gas. Most simply don't look beyond that to a per mile cost where the two level out.
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Old 01-28-2014, 01:05 PM   #13
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Diesel engines are power rated at the "smoke point," which is where visible smoke starts coming out the exhaust. That smoke is unburned fuel in the form of soot, and marks the point where the economy of a Diesel starts dropping off. If one is willing to tolerate the smoke and loss of economy a Diesel can make substantially more power.

NOx is another issue. Diesels are efficient largely because of their high compression (actually, expansion) ratios. That causes quite high temperatures and pressures in the combustion chamber. Diesels cannot burn all their air. The nitrogen and oxygen under the high pressures and temperatures combine to form any of about five nitrogen oxides. The only way to avoid this NOx formation is to reduce the temperatures and pressures, often done by changing fuel injection timing or by exhaust gas recirculation. Both techniques reduce efficiency. This is also the reason for EGR in gasoline engines. In gasoline engines some of the NOx can be reduced by proper catalytic converters, but I don't know that Diesel exhaust conditions allow that to happen.

Sorry, folks, but clean air has its costs.

As far as lubricity in fuels, the only place that matters in those parts which handle fuel, such as pumps and injectors. It makes no difference at all in the crankcase, wherein lives the oil that is periodically changed.
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Old 01-28-2014, 02:29 PM   #14
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The diesel modders don't have to tolerate the smoke. It's behind them, and the impression I get is that they don't care if others tolerate it or not. New diesels are harder to modify, and the cost of a new DPF likely will have them stopping on daily drivers.

A long expansion stroke is also why a premium gasoline, Atkinsoned, and direct injected engine are more efficient. Diesels get a further efficiency boost by not having a throttle plate. When a throttle plate is partially open, the engine has to suck harder to draw in air. That and other associated affects are referred to as pumping losses. Diesels control fuel flow instead of air flow to control engine speed. So they don't need a throttle plate that even wide open restricts air flow to a degree. It's the difference between using a narrow and wide straw with a shake.

The lack of throttle lets all the air in that the diesel needs burn. At idle and low demands means that there is more oxygen than needed to burn all the fuel in the cylinder, or less fuel to burn all the O2, thus why it is known as running lean. This great from an efficiency stand point. You don't risk as much fuel not getting burnt due to a lack of oxygen. The Civic VX and other fuel misers of the past used lean burn with gasoline for this reason.

The reason gasoline cars no longer do this is because of the formation of NOx from the unburnt air. Gasoline engines' throttle plate actually helps here. The ECM controls the amount of fresh air entering the cylinder so that there is just enough oxygen to burn the amount of fuel being injected. Diesels now have an EGR system that helps in controlling the available oxygen, but can only do so much.

Now that there is ULSD available, a three way catalytic converter can now be used with diesels. Previously, the sulfur would poison the catalyst for NOx reduction. They still produce more NOx than gasoline engine before the cat though. A NOx trap is one solution. These bound up the NOx during times, such as idling, when more is produced, and then slowly releases it so that the cat handle it.

The other way is to use SCR(selective catalyst reduction). This is the system that uses urea solutions such as AdBlue. Generically, they are known as DEF(diesel exhaust fluid). It adds to operating costs, but is quite low for the benefits. As long as you don't buy it from a Merecedes dealer.

The plus with SCR is that it doesn't reduce fuel efficiency like an EGR alone or NOx trap can. Compare the TDI Jetta and Passat from a couple years ago. The Jetta didn't have SCR then and the Passat did. Despite being larger and heavier, the Passat has nearly identical EPA ratings to the Jetta. Nearly all new passenger diesel vehicles in the US are using SCR.
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Old 01-31-2014, 01:56 PM   #15
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You guys lost me a bit there. It just seems a little mad that a 7.0 litre diesel truck is acceptable in America, even though it uses up to 1000% more fuel, and emits 500% more carbon than the best Europe can offer, regardless of any regulations. Everything is so "different" in the US, trying to compare things directly is near impossible. Similar to safety regulations, some European cars need beefing up for the US market, and yet US cars score badly in the Euro Ncap tests (new Jeep Compass got 2 out of 5 stars for example)
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