Why are EMUs so much more reliable than DMUs?

[SolomonSouth]

Based on the 2019-20 figures here: https://www.modernrailways.com/article/golden-spanners-2019-return-improvement

The most reliable EMU is the 387s on c2c (which of course are with GWR now) at 206000 miles between failures approx.
Yet the most reliable DMU (the Class 185) at approx 27,000 miles between failures would be only about halfway through the first generation EMU table between the 377/5 and the 465/9.

Can anyone tell me why EMUs tend to be much more reliable than DMUs?

 

[swt_passenger]

Based on the 2019-20 figures here: https://www.modernrailways.com/article/golden-spanners-2019-return-improvement

The most reliable EMU is the 387s on c2c (which of course are with GWR now) at 206000 miles between failures approx.
Yet the most reliable DMU (the Class 185) at approx 27,000 miles between failures would be only about halfway through the first generation EMU table between the 377/5 and the 465/9.

Can anyone tell me why EMUs tend to be much more reliable than DMUs?

Because Diesel engines and all their ancillaries just have far more to go wrong.

 

[ComUtoR]

Based on the 2019-20 figures here: https://www.modernrailways.com/article/golden-spanners-2019-return-improvement

The article is behind a paywall.

 

[pdeaves]

Based on the 2019-20 figures here: https://www.modernrailways.com/article/golden-spanners-2019-return-improvement

The most reliable EMU is the 387s on c2c (which of course are with GWR now) at 206000 miles between failures approx.
Yet the most reliable DMU (the Class 185) at approx 27,000 miles between failures would be only about halfway through the first generation EMU table between the 377/5 and the 465/9.

Can anyone tell me why EMUs tend to be much more reliable than DMUs?

swr_passenger is right. An electric unit has far fewer moving parts to look after.

 

[option]

Solid-state electronics, because they have no moving parts, are reliable. They will just do the same things over & over & over.

Mechanical drives, especially when having changes imposed on them that change stresses, subject the materials they are made from to mechanical stresses, which can weaken & then break the material. (There can also be a wider range of thermal stress)


One of the few moving parts on an EMU is the motor, & motors are very reliable.
A DMU has lots of moving parts in the engine, then moving parts in whatever system is used to transfer power to the wheels.


Generally, we tend to repair mechanical items, whereas we replace electronic components.
So, a replacement part can be surrounded by parts that have been subject to the same stresses, but not been changed.
If a chip/component goes wrong, it's often easier to replace the whole board it's on, & then there's a whole section of new parts.

 

[RT4038]

swr_passenger is right. An electric unit has far fewer moving parts to look after.

This phenomenon applies to diesel buses vs. trolleybuses and tramcars too.

However, to make a sensible comparison, you need to consider the reliability of the traction system as a whole - diesel fuel delivery vs. electricity supply delivery, in which I suspect the former is much more reliable than the latter. Aside from the rare 'panic buy' shortages or a breakdown in the pump, there is little to go wrong, compared to overhead wire/conductor rail and associated issues in the electricity supply

 

[AM9]

This phenomenon applies to diesel buses vs. trolleybuses and tramcars too.

However, to make a sensible comparison, you need to consider the reliability of the traction system as a whole - diesel fuel delivery vs. electricity supply delivery, in which I suspect the former is much more reliable than the latter. Aside from the rare 'panic buy' shortages or a breakdown in the pump, there is little to go wrong, compared to overhead wire/conductor rail and associated issues in the electricity supply

I would suggest that taken holistically, EMUs and their power supply are still more reliable than DMUs. The fact that EMUs are generally deployed on far more heavily used infrastructure than DMUs means that power failures get bursts of bad publicity for a shorter time. A diesel failure usually means that a replacement is needed, (which often isn't available).

 

[APT618S]

However, to make a sensible comparison, you need to consider the reliability of the traction system as a whole - diesel fuel delivery vs. electricity supply delivery, in which I suspect the former is much more reliable than the latter. Aside from the rare 'panic buy' shortages or a breakdown in the pump, there is little to go wrong, compared to overhead wire/conductor rail and associated issues in the electricity supply

I agree. It would be interesting to see the total delay minutes caused by EMU failures + overhead/3rd rail failures vs DMU failures. An overhead dewirement can cause total delay minutes an order of magnitude higher than a DMU failure.

 

[swt_passenger]

I agree. It would be interesting to see the total delay minutes caused by EMU failures + overhead/3rd rail failures vs DMU failures. An overhead dewirement can cause total delay minutes an order of magnitude higher than a DMU failure.

The statistics Modern Railways report on are only to compare similar fleets and by TOC performance, I’d have thought supply side problems would be ignored as being an issue for Network Rail?

 

[RT4038]

I would suggest that taken holistically, EMUs and their power supply are still more reliable than DMUs. The fact that EMUs are generally deployed on far more heavily used infrastructure than DMUs means that power failures get bursts of bad publicity for a shorter time. A diesel failure usually means that a replacement is needed, (which often isn't available).

I really don't know. I live at a major junction which is served by lots of electric and few diesel trains. However, power failure/lines down or defective/pantographs tangled or power switched off for various reasons seems to happen with relative frequency, affecting every train for miles around. I am sure Diesel units overall conk out more often, but not with such disruptive effect.

That is not to say one or the other are 'better', just that a comparison needs to take into account all the factors.

 

[357]

DMUs are not affected by supply problems or overhead wire problems, however.

And the c2c 387s are a microfleet that only work during morning and evening peak. For most of last year they had 0% or 50% fleet utilisation.

 

[RT4038]

The statistics Modern Railways report on are only to compare similar fleets and by TOC performance, I’d have thought supply side problems would be ignored as being an issue for Network Rail?

Quite.

 

[Bikeman78]

Solid-state electronics, because they have no moving parts, are reliable. They will just do the same things over & over & over.

Mechanical drives, especially when having changes imposed on them that change stresses, subject the materials they are made from to mechanical stresses, which can weaken & then break the material. (There can also be a wider range of thermal stress)


One of the few moving parts on an EMU is the motor, & motors are very reliable.
A DMU has lots of moving parts in the engine, then moving parts in whatever system is used to transfer power to the wheels.


Generally, we tend to repair mechanical items, whereas we replace electronic components.
So, a replacement part can be surrounded by parts that have been subject to the same stresses, but not been changed.
If a chip/component goes wrong, it's often easier to replace the whole board it's on, & then there's a whole section of new parts.

Even older EMUs are considerably more reliable than their equivalent DMUs. A class 150 and a class 317 are basically the same, especially if you compare a 150/2 with a 317/6. The electric is about twice as reliable as the DMU. Presumably the common equipment between the two classes has a similar failure rate so all the extra failures on the class 150s are likely to be engine faults. Often caused by coolant or oil leaks.

 

[AM9]

DMUs are not affected by supply problems or overhead wire problems, however.

That is two different things. Whilst a supply problem might temporarily affect the reliability of services, a failure of the rolling stock generally involves a return to depot, an investigation and possibly an extended wait for a fix before a return to service. That is why for a given service, more DMUs than EMUs are needed, to cover for servicing and repair absences.

 

[SolomonSouth]

Thanks for all the responses by the way.

Solid-state electronics, because they have no moving parts, are reliable. They will just do the same things over & over & over.

Mechanical drives, especially when having changes imposed on them that change stresses, subject the materials they are made from to mechanical stresses, which can weaken & then break the material. (There can also be a wider range of thermal stress)


One of the few moving parts on an EMU is the motor, & motors are very reliable.
A DMU has lots of moving parts in the engine, then moving parts in whatever system is used to transfer power to the wheels.


Generally, we tend to repair mechanical items, whereas we replace electronic components.
So, a replacement part can be surrounded by parts that have been subject to the same stresses, but not been changed.
If a chip/component goes wrong, it's often easier to replace the whole board it's on, & then there's a whole section of new parts.

Sums it up very well. Thank you.

Because Diesel engines and all their ancillaries just have far more to go wrong.

Simplistic but also a good response.
Cheers guys.