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Why aren't more car companies to using a diesel-electic powertrain? They've been used in the locomotive industry for decades!

duaneb 14y ago
Pressure from gas companies?
eric-hu 14y ago
Mono-nitrogen oxides, aka NOx, are a regulated byproduct of diesel and gasoline engines. They're produced especially at high combustion temperatures and are a component of acid rain. NOx itself is pretty toxic to humans too. Normal modern diesel engines are unthrottled [1] and control the power output of the engine by limiting the amount of fuel injected to the engine. The unthrottled bit is important--we'll come back to that. For now, just note that the engine computer can't regulate how much oxygen gets into the combustion chamber. By comparison, modern gasoline engines use a throttle to control how much oxygen gets into the combustion chamber. On top of that, the fuel injection is computer-controlled to deliver enough gasoline to the engine so that all oxygen is consumed and there's some left-over gasoline from the combustion. That leftover gasoline plays an important role in temperature control--liquid matter tends to have much more specific heat than gaseous matter. The gasoline absorbs some of the heat released from the combustion such that combustion chamber temperatures don't spike, creating excessive NOx. Back to diesel engines, because the basic design of diesels uses no throttle, the engine will have minimal post-combustion liquid droplets to absorb the excess energy (heat) that would cause massive NOx emissions. There are workarounds: I. Some diesel engines introduce throttles for this purpose [2], but adding a throttle to an engine introduces pumping losses to the engine. Being throttleless is one of many reasons why diesel engines tend to be more efficient than equivalently sized gasoline engines. This system tends to cost more because of the extra engineering involved. II. Diesel exhaust fluid [3] can be sprayed into the exhaust stream of diesel engines. DEF is pretty much urea and purified water. The urea combines with NOx to form ammonia and CO2. A selective catalytic reducer (SCR) downstream converts the ammonia into water and N2. This system is somewhat simple and clean, but is open to lazy and cheap owners. I've been told that this system is disallowed in America for consumer vehicles. The DEF is consumed fairly quickly and needs to be refilled at least as frequently as oil changes. There's no reasonably cheap way to regulate that cars have adequate DEF, as the engines will run just fine without it. The only side effect to a diesel engine out of DEF is that it'll output NOx like your typical 1970's car in Los Angeles. Owners have little incentive (beyond moral obligation) to refill DEF according to a schedule. They might even be ignorant as to the side effects. So long story short, producing consumer diesels that meet regulations is pretty hard. The freight industries are not as regulated regarding diesel (maybe land-shipping industries are more tightly regulated?...there are holes in my knowledge). [1]: http://en.wikipedia.org/wiki/Diesel_engine http://en.wikipedia.org/wiki/Diesel_engine [2]: http://en.wikipedia.org/wiki/Throttle#Internal_combustion_engines http://en.wikipedia.org/wiki/Throttle#Internal_combustion_en... [3]: http://en.wikipedia.org/wiki/Diesel_exhaust_fluid http://en.wikipedia.org/wiki/Diesel_exhaust_fluid

duaneb 14y ago

Pressure from gas companies?

eric-hu 14y ago

Mono-nitrogen oxides, aka NOx, are a regulated byproduct of diesel and gasoline engines. They're produced especially at high combustion temperatures and are a component of acid rain. NOx itself is pretty toxic to humans too. Normal modern diesel engines are unthrottled [1] and control the power output of the engine by limiting the amount of fuel injected to the engine. The unthrottled bit is important--we'll come back to that. For now, just note that the engine computer can't regulate how much oxygen gets into the combustion chamber. By comparison, modern gasoline engines use a throttle to control how much oxygen gets into the combustion chamber. On top of that, the fuel injection is computer-controlled to deliver enough gasoline to the engine so that all oxygen is consumed and there's some left-over gasoline from the combustion. That leftover gasoline plays an important role in temperature control--liquid matter tends to have much more specific heat than gaseous matter. The gasoline absorbs some of the heat released from the combustion such that combustion chamber temperatures don't spike, creating excessive NOx. Back to diesel engines, because the basic design of diesels uses no throttle, the engine will have minimal post-combustion liquid droplets to absorb the excess energy (heat) that would cause massive NOx emissions. There are workarounds: I. Some diesel engines introduce throttles for this purpose [2], but adding a throttle to an engine introduces pumping losses to the engine. Being throttleless is one of many reasons why diesel engines tend to be more efficient than equivalently sized gasoline engines. This system tends to cost more because of the extra engineering involved. II. Diesel exhaust fluid [3] can be sprayed into the exhaust stream of diesel engines. DEF is pretty much urea and purified water. The urea combines with NOx to form ammonia and CO2. A selective catalytic reducer (SCR) downstream converts the ammonia into water and N2. This system is somewhat simple and clean, but is open to lazy and cheap owners. I've been told that this system is disallowed in America for consumer vehicles. The DEF is consumed fairly quickly and needs to be refilled at least as frequently as oil changes. There's no reasonably cheap way to regulate that cars have adequate DEF, as the engines will run just fine without it. The only side effect to a diesel engine out of DEF is that it'll output NOx like your typical 1970's car in Los Angeles. Owners have little incentive (beyond moral obligation) to refill DEF according to a schedule. They might even be ignorant as to the side effects. So long story short, producing consumer diesels that meet regulations is pretty hard. The freight industries are not as regulated regarding diesel (maybe land-shipping industries are more tightly regulated?...there are holes in my knowledge). [1]: http://en.wikipedia.org/wiki/Diesel_engine http://en.wikipedia.org/wiki/Diesel_engine [2]: http://en.wikipedia.org/wiki/Throttle#Internal_combustion_engines http://en.wikipedia.org/wiki/Throttle#Internal_combustion_en... [3]: http://en.wikipedia.org/wiki/Diesel_exhaust_fluid http://en.wikipedia.org/wiki/Diesel_exhaust_fluid