5 ms·

IIRC basic thermodynamics shows that ordinary steam engines can't be very efficient. So, don't get good MPG or miles per bucket of coal, or whatever fuel. IIR

IIRC basic thermodynamics shows that ordinary steam engines can't be very efficient. So, don't get good MPG or miles per bucket of coal, or whatever fuel. IIRC the main reason for diesel-electric locomotives for trains and large ships was that it was significantly more efficient than just steam, at least steam with just a boiler and pistons. Of course, really large ships generate steam and then run it through a steam turbine -- no Diesel around.

dsfyu404ed 7y ago
For trains diesel electric lets you really simplify the power-train. Imagine powering all those wheels mechanically, yeah, not happening.
- jcranmer 7y ago
  The most common diesel-electric locomotive today has 6 drive axles. The big steam locomotives designed at their twilight sported 6-10 drive axles, and the most famous and successful of them (the 4-8-8-4 Big Boy) ran 8 drive axles. Mechanical linkage of drive axles isn't what killed the steam locomotive.
  - dsfyu404ed 7y ago
    Mechanical linkage for multiple direct drive external combustion engines that can operate at 0 rpm is far simpler (the drive axles are basically the crankshaft) than for a single internal combustion engine that must maintain a minimum engine rpm and has to be geared down to axles that are riding on a bogie that pivots separate from the deck of the engine. To take advantage of the increased efficient of an internal combustion engine you'd need a lot of mechanical complexity unless you go hydraulic (solidly defeating most efficiency improvements) or electric.
nradov 7y ago
Stream engines are no longer used on large ships (except for nuclear powered warships). Merchant ships now use large 2-stroke diesel engines for maximum fuel efficiency.
- graycat 7y ago
  Yup, the nuke ships are what I was thinking about -- wasn't sure if there were any other examples of pushing steam through a turbine.
  - nradov 7y ago
    There used to be many large ships driven by oil-fired steam turbines but those have gradually disappeared over the past few decades. Compared to diesel engines, steam turbines are less efficient and require more maintenance.
iguy 7y ago
> basic thermodynamics shows that ordinary steam engines can't be very efficient No this is not true, see other comments / Carnot cycle. Steam can be extremely efficient. But diesel engines are much simpler and more compact, it's a trade-off.

dsfyu404ed 7y ago

For trains diesel electric lets you really simplify the power-train. Imagine powering all those wheels mechanically, yeah, not happening.

jcranmer 7y ago
The most common diesel-electric locomotive today has 6 drive axles. The big steam locomotives designed at their twilight sported 6-10 drive axles, and the most famous and successful of them (the 4-8-8-4 Big Boy) ran 8 drive axles. Mechanical linkage of drive axles isn't what killed the steam locomotive.
- dsfyu404ed 7y ago
  Mechanical linkage for multiple direct drive external combustion engines that can operate at 0 rpm is far simpler (the drive axles are basically the crankshaft) than for a single internal combustion engine that must maintain a minimum engine rpm and has to be geared down to axles that are riding on a bogie that pivots separate from the deck of the engine. To take advantage of the increased efficient of an internal combustion engine you'd need a lot of mechanical complexity unless you go hydraulic (solidly defeating most efficiency improvements) or electric.

nradov 7y ago

Stream engines are no longer used on large ships (except for nuclear powered warships). Merchant ships now use large 2-stroke diesel engines for maximum fuel efficiency.

graycat 7y ago
Yup, the nuke ships are what I was thinking about -- wasn't sure if there were any other examples of pushing steam through a turbine.
- nradov 7y ago
  There used to be many large ships driven by oil-fired steam turbines but those have gradually disappeared over the past few decades. Compared to diesel engines, steam turbines are less efficient and require more maintenance.

iguy 7y ago

> basic thermodynamics shows that ordinary steam engines can't be very efficient No this is not true, see other comments / Carnot cycle. Steam can be extremely efficient. But diesel engines are much simpler and more compact, it's a trade-off.