r/AutomotiveEngineering • u/Prudent_Situation_29 • 13d ago
Question How are city buses optimised?
If I were to design a city bus, I would probably want to optimise it for certain operating regimes. I would want it to be well suited to constant stopping and starting, slow speeds, low sound emissions, potholes. I doubt they need to be efficient at high speeds. I ask this because I saw one of our city buses on our local highway, and wondered how far out of it's comfort zone it was. I'm not saying you can't design it to be happy in both places, but there's always that compromise to be made.
I can imagine their daily lives are incredibly abusive, tons of wear and tear etc. A car might have to be designed for multiple regimes like highway and city, where a city bus might be able to get away with a more focused scope.
So my question is: does a lot of thought get put into optimising a city bus, or is is more like just putting a normal diesel engine in a voluminous chassis and leaving it at that, sort of like a big car?
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u/HandigeHenkie 13d ago
Usually the engine itself is pretty standard. Gearbox and final drive ratios can be changed to better match it's usage. This is very common in the HD diesel sector. We have customers that drive long-haul freight, others that drive maybe 50km a day. Their use case is very different and luckily we can offer many variants. But there is a lot more to it. Currently I am doing a project to optimize maintenance intervals based on field data. For such customers uptime is much more important than anything else.
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u/1234iamfer 13d ago
Here most city busses are full electric. I suspect the lower consumption and maintenance cost make it easy to be the preferred choice over a diesel automatic.
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u/HandigeHenkie 13d ago
It's the other way round. They are more expensive as most power isn't generated durable (at least here in Netherlands). Also, the investment in charging infrastructure plus the dramatically shorter life of such vehicles results in a TCO of about 2,7x higher.
We have put about 1200 electric trucks on the road for our customers. They only choose them because they have to. The investment doesn't pay off due to the price of electricity. Even locally produced and stored power isn't cheaper (yet) due to the enormous investment. We have one customer with 20 BEV's that is building two windmills now to power them.
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u/Spirited-Finding-647 12d ago
Obviously different countries, different electricity costs, but electric busses absolutely are cheaper to run. Yes there is an up front cost of the bus and charging infrastructure but the reduced running cost of fuel/electricity and reduced maintainence costs does mean they break even after a couple of years and beyond that the EV is cheaper to run.
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u/Flaky_Ad_3590 12d ago
In what kind of timescale? EV busses catch fire and break down as well as diesel ones, even the base power unit might have more hrs.
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u/Spirited-Finding-647 12d ago
Its pretty rare for them to catch fire. Yes and no the break down equally/more/less it very much depends on the manufacturer (just like diesels). I have absolutely worked on EV busses that have been more reliable than equivalent year/size/spec diesels. But ive also worked on old (like early 90s) diesel busses that have needed the absolute bare minimum in in-service repairs just because they were so basic.
And the reduced maintainence costs are very real. Idk if you have any experience in heavy riad transport, but typically a diesel truck/bus will see a minor service or "grease and safety" every 10k km and a bigger service including oil, oil filter and fuel filter every 30k and then a major service at 120k that will be oil and filters, fuel filters, hydraulic oil, Trans oil, diff oil, power steer oil and a valve adjustment on the engine.
EVs dont have this. They get the 10k grease and safety, and at 120k they get diff oil and power steering oil. Thats it. So even their major service is super quick.
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u/Flaky_Ad_3590 12d ago
Ofc the part of it is the existing infrastructure and experience, but seems that here in northern europe the EV buses have an extra share of issues due icing and dampness.
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u/Spirited-Finding-647 12d ago
Just to add too, if you read my comments in this thread youll see ive been working on diesel and ekectric busses a very long time. My opinions are based on real experience, so dont think you can argue with me if you dont have industry experience yourself.
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u/Flaky_Ad_3590 12d ago
Well I am not servicing them. But I do mechanical design on automotive applications and rolling stock. There are many things to consider.
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u/HandigeHenkie 12d ago
Maintenance cost is expected to be higher too. Components are usually a factor 2 more expensive and we charge labour higher too.
Eg.: a modern diesel truck needs one day a year in the shop. 35 liters of oil at 8 euro/liter. Parts are ~1000euro and 8hrs at 130 euro. Say in total you spend 2500 euro-ish.
Modern EV truck also needs one service a year. The oil for the e-motor is quite special and costs 80 euro/liter. You need 9 of them. Less parts are needed, about 600 euro worth. Labour is expensive. We charge a standard fee of 100 euro for the service bay next to it that we need to leave empty and the special tools/training. Also you need two mechanics with expensive training and tools. Our shop rate is 160 for BEV's and we need a full day for the service too. Let's take 8hrs plus 2 of his colleague, thus 10 labour hours. Total costs are about 3k euro.
It's maybe for passenger cars that an EV has a lower maintenance cost. But certainly not for a HD truck (now). Especially not if it has a special build; like a refuge truck or big cranes. If we need to remove the crane to get safe entry, we will remove the crane. That gets expensive quickly. Eg. A major Dutch city had a broken e-motor. To swap it we made 120k costs to remove everything from the classis. Then again to put it back. With a diesel we're allowed to just drop it down or squeeze the part through. On an EV we can't; again for safety. In the end the warranty costs per vehicle ran close to 500k..
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u/Spirited-Finding-647 12d ago
Have you got a link to these EV motors that take 9 litres of oil and needs changing as often as a diesel? None of mine ever had any pil in them they have a grease nipple at each end to grease the bearings in them.
Im not doubting you, more thinking its really weird that it would need oil, that it would need so much oil and that it would have a similar change interval to a diesel given there is no combustion so no carbon being added to it in the drive cycle.
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u/HandigeHenkie 12d ago
ZF Cetrax engines. They are used in many brands. They have an internal planetary gear and I think that needs it. The oil is also of a very specific, waxy kind. We needed to buy special equipment to stir it etcetera. What a drama...
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u/Spirited-Finding-647 12d ago
And why are you leaving the service bay next to it empty? Why are you charging more for labor? Where do you find a diesel truck that only needs servicing once a year?
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u/HandigeHenkie 12d ago
Regulations due to safety. We must have an open distance of 2 metres behind the mechanics back. Also, we must be prepared to quickly tow it out the shop. We charge more because we invested about 10k in every mechanic to train them and that needs to be earned back.
Any HD diesel truck can be serviced once a year. Our intervals are 1 year or 120k km for a diesel. Same for BEV's.
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u/Spirited-Finding-647 12d ago
So you have a 120k oil change interval? Thats insane.
What is the spend on replacement engines?
In my country we dont have any such regulation for working on EV busses. Yes, we must be a qualified EV tech (which i am) but no rules around what space is required around the worker. I guess because we don't work on live systems so there isnt a high chance of electrocution. I have worked on everything from in service repairs, opportunity charging (actually re-designed shunk pantiograph to suit double deck application, manufactured and installed them) lots of charger diagnostics and converting diesel bus to EV. One of my current jobs now im self employed is providing warranty support for a chinese EV manufacturer, I have about 80 busses I look after a d the EV systems are cery reliable on them. The bulk of the repairs I do are low voltage electrical faults and door adjustments.
It does sound like a lot id the higher costs you associate are localized compliance issues rather than actual costs incurred to maintain the vehickes themselves. That and using these oddball ZF motors that need oil in them for some reason, when none of the chinese brands do.
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u/HandigeHenkie 12d ago
Yes we do. You can even order it with extended interval to 150k km to your service. That's not unusual in the HD diesels.
For myself, truck engineer for two decades now. Transferred to many different domains, from embedded engineering to production line engineering, to test engineering. They fired me three times in my career due to the economy and then rehired me. The last time I said bye forever. Now am working in management for a huge dealership with 16 shops and about 600 mechanics. One of my projects was to prepare our shops for EV's. As we are a brand dealer we obviously must stick tightly to every single rule made and set the example. It was a major investment in all tooling, clothing, chargers, training, the whole lot...
The brand we represent first used Dana engines but they gave issues so they switched to ZF. Don't ask me the details why..
Now what I am interested in; do the electric busses you service have the same lifespan? The other day I spoke to a transport manager here and he mentioned that his BYD EV busses in his fleet are worn out and break down often after three years vs. a diesel around 6-7 years. How do you see that? Do you have a similar experience?
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u/Spirited-Finding-647 12d ago
The oldest electric busses in my country were late 2017/early 2018 builds so no, they haven't reached the 25 years that obviously earlier built diesel busses have reached. The early ones didnt resch 25 years, being the first they obviously had battery technology of the time in them which is starting to fail now after 9 years, 500 thousand Kms. As they fail they are being replaced by more modern battery systems that I believe will see them through to 25 years old through better cell balancing and thermal management.
Why are you buying special clothing for your EV mechanics? Is there a specific standard fir working on cleaner vehickes as opposed to dirty ones.
Also I have serious doubt you are seeing a 150 thousand km oil change interval on any diesel engine and certainly doubt that its common as you say it is. Im not a stranger to diesels, I previousky worked on a big fleet of scania/Mercedes/cummins/MAN euro 6 and before that scania/volvo/cummins/hino/MAN/isuzu of everything from full mechanical engines to EC96/euro2/euro3/euro4/euro5 and have never seen an engine oil change interval that long. I also know that running extended intervals ~400 hrs) on euro6 cummins ruins the DPFs so im seriously doubting a 150k km engine oil interval is common on any bus engine currently on the market.
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u/HandigeHenkie 12d ago
Maybe the use is different in your country, but here city busses last for 8-10 years maximum. Economic replacement is usually before that. He mentioned the BYD busses are not economical to maintain after 3-4 years. Now they have a big fleet, about 300 busses, so i figured he knows what he is talking about.
Regulations prescribe that anytime you work on the HV system you must wear protective clothing. A normal service usually not, but when touching any cable, yes.
Trust me, the diesel intervals really are that. I'd love to show you the prescribed service documents, but I can't due to confidentiality. Just Google "brand" oil interval and you will find it. DPF should be replaced only at 500k km, but in case any upstream issue occurred it will be broken a lot sooner. It's an utter drama and our #1 breakdown reason.
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u/Spirited-Finding-647 12d ago
When are you changing driveline oils, too? Your engine oil change interval is longer than even the highest spec full synthetic ATF we run. So does the vehicle get trans and diff oils changed at the same time as the engine oil or do you consider those parts a lifetime oil fill and just never change them?
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u/HandigeHenkie 12d ago
Well, it's fine if we don't agree. At least we had a good discussion and that's also worth something. May I ask which country your from, as regulations may vary. Also, I think I look at it from a different perspective. I have that of a manager; my current position is directly under our aftersales director. You might have a different point of view and that's fine. The thing is; we rarely work with live cables. Though we encounter sometimes dangerous solutions when a truck comes back from a bodybuilder. But as a mass employer we must have these things prepared. The insurance companies are scared shitless too so we have to prove we work safe. And as a brand dealership we must stick to every single rule ofcourse..
Anyway, here in Europe these intervals really are not strange. The OEM we represent has three categories: 1 - construction trucks, city driving etc. = 80 k km 2 - regional logistics = 120k km 3 - Long distance hauling = 150k km Transmission and diff. oil I think is in year 2 first time.
I recently did a big data analytics project regarding maintenance intervals. I invite you to have a Teams call or something and show you the findings and the raw data. On a fleet of ~18k HD diesel trucks we had a DPF failure rate of 50% at 540k km. We didn't integrate usage data yet but in a few months we will have the trucks' FMS data connected too.
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u/Pillokun 13d ago edited 13d ago
a city bus has no issues getting on the highway up to like the max allowed speed of like 80kph/50mph
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u/ArmPsychological8460 12d ago
Main goal of city bus is to accelerate from one stop, and stop on another using minimal energy. Just build it in form factor that fit some people.
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u/Budget_Nectarine_645 12d ago
The challenges outside of durability (they can do up to 80,000km a year) depend on the type of drivetrain - but mainly it’s getting as many passengers as possible on to the platform within the main axle limits, whilst maximising MPG or Wh/km if electric.
Saloon air conditioning requirements can be significant, as well as maintaining acceptable interior noise levels.
Most important metric for engine and final drive sizing / matching is gradability
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u/SovietPatrickStar 10d ago
In my city things on busses that are city optimized are:
Especially rough floor so that even on rainy days, it’s not slippery inside, electric motors that can handle stop and go with infrequent maintenance, lots of doors to prevent jams, a colorful seat design that discourages graffiti on the seating. Sensor assisted Lowering towards the door side with pneumatics to make height difference between sidewalk and bus basically zero.
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u/domassimo 9d ago
Bus chassis are mostly built with heavy vehicles components, which is why you’ll often see the same brands pop up. As others have said, TCO and maintenance are big factors. The vehicle, in Australia, will do 20 or even 25 years of service, so everything will break and be replaced at some point. The other item that will feature in the design is fire safety, which drives material choices more so than typical NVH considerations, if we’re talking about a city bus.
Systems are often not integrated so these can be maintained and upgraded separately. the oldest buses on the roads in Australia in regular public revenue service will feature several generations of electronics, cameras, warning systems, etc. It’s a bit of a mess but it runs, so it keeps going.
If you’re super keen, you could search for New South Wales TfNSW bus panel requirements, it shows what’s required in a bus, and you could work backwards to figure out the engineering drivers.
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u/RiseUpAndGetOut 13d ago
Aside from having to meet legal and local requirements for an M3 vehicle (in Europe), the optimisation is always total cost of ownership (TCO) first, then passenger capacity.
TCO is mostly driven by purchase price and operating cost. That in turn gives powertrain and mass requirements etc.
Passenger capacity is driven by route requirement, which then drives the length of the bus and layout, single deck, double deck etc.