European Vs. North American Hybrids (1 Viewer)

trainsboy335

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I've got a question for the bus community out there. I am a bus operator in the US and have been driving a lot of hybrids lately. I've noticed that while hybrid buses in the US tend to lean more toward the CVT (continuously variable transmission) side, having almost no distinct gear shifts, whereas European hybrid buses tend to lean towards a geared system (Volvo's iShift comes to mind). Why is this? Is there a difference when it comes to degree of "hybridization" or is it more preferential? The best example of how the ones I drive tend to work goes like this: From 0 to about 20 MPH, the electric motor does all the work, from about 20-40 MPH the ICU (internal combustion engine) is coupled to the driveshaft and works in tandem with the electric motor, from 40 MPH on up, its purely the ICU. The ICU then tends to rev when braking from higher speeds as well, presumably to help recoup more power for the batteries. I'd love to hear possible answers for why there is a difference or even opinions as to why the two regions have differences with hybrids :)
 

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Road-hog123

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Volvo's I-Shift is not a hybrid system, it's a gearbox similar to an American 18-speed manual truck gearbox. The gear stick and clutch are replaced by pneumatic actuators and the operator's brain with a computer and sensors. It's available in Volvo trucks with a large number of options including additional crawler gears, dual-clutches and hybrid drive.

Volvo hybrid buses use the same hybrid system as Volvo trucks, but with a basic I-Shift gearbox with no options. Between the clutch and gearbox is an Integrated Starter Alternator Motor (I-SAM) which can be used to start the engine (only while in neutral), charge the batteries and drive the wheels. When starting off, the computer picks a suitable gear and sets off with just the electric motor. At the end of that gear (or if more power is required), the engine is started and further driving is performed with both the diesel engine and electric motor. Braking is supplemented with the electric motor to recharge the batteries. When the handbrake is applied the engine will usually stop, but if there's not enough battery charge the engine will keep running, driving the motor while in neutral to charge the batteries.

Your system sounds quite similar, except it doesn't use a transmission with noticable gaps. The engine revving up when braking is not to help charge the batteries but to provide engine braking, which is not uncommon on conventional diesel vehicles to reduce brake wear. I would guess that the electric motor is after the gearbox and has a maximum speed of about 450 rpm, so stops being effective as either a motor or brake around 40 mph. It's interesting you describe it as a CVT as they're not very common in buses, at least over here - perhaps you could provide more info about the make/model so I could look it up?

Non-Volvo buses tend to use the same system as yours - essentially a conventional diesel bus with an electric motor for low-speed driving. I would imagine they take the place of a retarder (turbine that spins in oil to slow the vehicle without using the brakes), hence the need for engine braking at high speeds.

These hybrid systems are "parallel" hybrids - both the engine and electric motor drive the wheels mechanically. The New Routemaster (NRM) in London uses Siemens' ELFA2 hybrid system, which is a "series" hybrid - the engine is just a generator, with the electric motor doing all the driving. These buses have no gearbox at all - the engine runs when required, independently of road speed. This system is used in a number of buses on the continent too, but I don't think series hybrids are as common as parallel hybrid vehicles.

As for why different countries use different systems, it's up to the operators which buses they buy, and manufacturers which system they use. Volvo already had a truck hybrid system, so it made sense to use it in their buses. It has gaps between gears (especially on earlier vehicles that used the I-SAM as a starter motor), but the "manual" transmission is better for fuel economy. Other manufacturers just modified their existing conventional diesel buses because it was easier, cheaper and faster to develop. Some realised that they'd need to make electric buses in the future anyway, so they got a head-start by making electric buses with generators until battery technology caught up - some early electric buses had enough capacity to drive the bus but not enough to also run the heating, so they had a diesel heater.
 
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trainsboy335

Member
15
12
Thanks for your post! It's really interesting! The bus I'm referring to is the Gillig 35ft Hybrid with an Allison EP40 (now H 40 EP) Split parallel-series hybrid system. I've always been told that the engine revving while braking is to help recharge the batteries however I wouldn't be surprised if it was a similar system to a locomotive engine and dynamic braking. When I take my foot off the gas or the amount of pressure I'm applying to the gas peddle is not sufficient to continue accelerating, the bus' computer flips a switch and activates regenerative braking to help slow the bus down (and let me tell you, it really slows the bus down). However, that being said, I have noticed that when the engine revs while braking, it begins slowing down even faster; even though the engine has been uncoupled from the drive shaft. I could potentially attribute this to the engine doing one or both of two things: 1.) helping to recharge the batteries after a large drain from accelerating; and 2.) braking taking the form of rheostatic braking, i.e., generating more electricity than the resistors can handle to place additional resistance on the electric motor. That's just my theory though. Here is a link to the product page for the Allison H 40 EP : Link
 
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trainsboy335

Member
15
12
Volvo's I-Shift is not a hybrid system, it's a gearbox similar to an American 18-speed manual truck gearbox. The gear stick and clutch are replaced by pneumatic actuators and the operator's brain with a computer and sensors. It's available in Volvo trucks with a large number of options including additional crawler gears, dual-clutches and hybrid drive.

Volvo hybrid buses use the same hybrid system as Volvo trucks, but with a basic I-Shift gearbox with no options. Between the clutch and gearbox is an Integrated Starter Alternator Motor (I-SAM) which can be used to start the engine (only while in neutral), charge the batteries and drive the wheels. When starting off, the computer picks a suitable gear and sets off with just the electric motor. At the end of that gear (or if more power is required), the engine is started and further driving is performed with both the diesel engine and electric motor. Braking is supplemented with the electric motor to recharge the batteries. When the handbrake is applied the engine will usually stop, but if there's not enough battery charge the engine will keep running, driving the motor while in neutral to charge the batteries.

Your system sounds quite similar, except it doesn't use a transmission with noticable gaps. The engine revving up when braking is not to help charge the batteries but to provide engine braking, which is not uncommon on conventional diesel vehicles to reduce brake wear. I would guess that the electric motor is after the gearbox and has a maximum speed of about 450 rpm, so stops being effective as either a motor or brake around 40 mph. It's interesting you describe it as a CVT as they're not very common in buses, at least over here - perhaps you could provide more info about the make/model so I could look it up?

Non-Volvo buses tend to use the same system as yours - essentially a conventional diesel bus with an electric motor for low-speed driving. I would imagine they take the place of a retarder (turbine that spins in oil to slow the vehicle without using the brakes), hence the need for engine braking at high speeds.

These hybrid systems are "parallel" hybrids - both the engine and electric motor drive the wheels mechanically. The New Routemaster (NRM) in London uses Siemens' ELFA2 hybrid system, which is a "series" hybrid - the engine is just a generator, with the electric motor doing all the driving. These buses have no gearbox at all - the engine runs when required, independently of road speed. This system is used in a number of buses on the continent too, but I don't think series hybrids are as common as parallel hybrid vehicles.

As for why different countries use different systems, it's up to the operators which buses they buy, and manufacturers which system they use. Volvo already had a truck hybrid system, so it made sense to use it in their buses. It has gaps between gears (especially on earlier vehicles that used the I-SAM as a starter motor), but the "manual" transmission is better for fuel economy. Other manufacturers just modified their existing conventional diesel buses because it was easier, cheaper and faster to develop. Some realised that they'd need to make electric buses in the future anyway, so they got a head-start by making electric buses with generators until battery technology caught up - some early electric buses had enough capacity to drive the bus but not enough to also run the heating, so they had a diesel heater.
Here is a video with an audio recording of what it sounds like (roughly, ours sound a bit different but it gets point across) Sound Transit 2012 Gillig Hybrid
 

Road-hog123

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Having looked into it, your initial description was correct - the key thing being that it is both a parallel and series hybrid so it can be both diesel-only above 40 mph and use the engine as a generator under braking. It's very innovative, and I shall definitely be looking into it further (hopefully I can find a cut-away drawing somewhere).

As for why it's not used over here, I have no idea. UK bus manufacturers were very busy failing at being innovative around the time the EP40/50 was introduced, had ADL offered it in their Enviro200, it might well have been widely used here.
 

trainsboy335

Member
15
12
That's very interesting to hear. We've had Allison transmissions like that for almost two decades. Some manufacturers offer a BAE hybrid system but I don't know too much about that one specifically.
 

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