Here’s why the all-new 2020 Honda City RS with i-MMD doesn’t need a gearbox
Hans · Aug 25, 2020 03:40 PM
Very soon, Honda Malaysia will be hosting the world debut of the first ever Honda City RS with the latest two-motor i-MMD full hybrid engine. The specs sheet, which will be released in due time, will say that the City drives its front wheels via an E-CVT gearbox.
In truth, the Honda City RS doesn’t have a gearbox, at least not a conventional one. However as you can imagine, it’s difficult to explain to customers that the car doesn’t have a gearbox, hence the E-CVT name.
There’s still the usual Park, Reverse, Neutral, and Drive modes on the shift-by-wire 'gear' selector, but none operates the usual drive belts of a conventional CVT, or multiple ratios of a conventional torque converter automatic transmission. There’s little need for torque multiplication because the electric motor (there’s two, but only one drives the wheels directly) is powerful enough to drive the car.
Honda says its i-MMD’s fixed ratio direct drive approach creates a direct connection between moving components, resulting in a smoother transfer of torque, and is more refined than planetary e-CVT typically found in other hybrid vehicles.
The e-CVT reference by Honda is aimed at Toyota, the only company to use planetary gear set e-CVTs, which like Honda's E-CVT name, doesn’t work like a conventional CVT at all, but that’s another story.
What’s so special about i-MMD hybrid?
Compared to the previous one-motor i-DCD, the two-motor i-MMD packs more power, and is also more fuel efficient. It was originally meant for upper segment cars like the Accord and CR-V.
As the technology matures, Honda is now able to package it tight enough to fit into a smaller cars like the Honda City.
What is a two-motor hybrid and why is it better?
Most hybrids, be it mild or full hybrid ones, use only one motor. This is the simplest, most cost effective setup. The engine can be shared with a standard combustion engine variant and the transmission only needs some modification to accommodate a motor, nestled between the engine’s crankshaft and the transmission.
One-motor hybrids work well enough but it’s not as efficient or as powerful (all else being equal) as a two-motor hybrid. With only one motor, the system can either drive the car, or recharge the batteries, but not at the same time. With two motors, it can.
Why i-MMD doesn’t need a gearbox?
The reason why conventional cars need a gearbox is because:
1. Combustion engines have a narrow operating range where peak torque is achieved, but since a car will need to travel at different speeds, going uphill and downhill, it’s impossible to have an engine driving the car at this fixed engine speed (rpm) range all the time, hence why gears are needed.
2. Torque multiplication. All engines have a quoted peak torque but the actual torque that’s delivered to wheels can be a lot higher if you pair it with different gear ratio combinations. This is how small engines are able to haul heavy loads.
Unlike engines, electric motors generate its peak torque immediately after 0 rpm. There’s no need to build up speed before peak torque is reached.
The Honda City RS makes 253 Nm and since it’s coming from an electric motor, peak torque is achieved from 0 rpm (OK technically it’s 0.1 rpm) until 13,300 rpm.
Where the motor tapers off, the 1.5-litre DOHC i-VTEC naturally aspirated combustion engine fills the torque gap, peaking at 127 Nm from 4,500 – 5,000 rpm.
Combining both drive sources, the car has enough torque to go uphill without needing lower gear ratios for torque multiplication, hence no need for a conventional gearbox.
i-MMD’s coupling clutch also allows the engine to be disconnected from the wheels and run at its most efficient operating range (most of the time).
How does it work its magic?
Below is a cross section of the i-MMD’s E-CVT, you will notice that it’s extremely compact.
There are two motors. The traction motor is the one that does the actual driving. It also doubles as a generator, reversing the flow of current when the car is coasting or braking, to charge the lithium-ion traction battery at the rear.
Located behind the traction motor is the generator, and this is where the added benefit of a two-motor setup comes in.
As it implies, the generator doesn’t drive the wheels. It’s spun by the engine, to generate electricity. Note that the generator doesn’t charge the traction battery, but only to provide additional power to the traction motor.
The 2-motor setup allows the engine to power the car without being directly connected to the wheels, thus allowing it to run at its optimal, most efficient rpm range. This is referred to as the Hybrid Drive mode, which is essentially a series-hybrid mode, but unlike Nissan e-Power, Honda’s i-MMD can also work in other modes as well.
At higher speeds, where it’s more efficient to use the engine, a clutch engages and the power flow is diverted away from the hybrid system, using the engine to drive the wheels directly (Engine Drive mode). When there is surplus power, the traction motor also works as a generator to charge the traction battery.
As a full-hybrid, the City RS will accelerate from stationary using electric power alone (EV Drive mode), thus doing away with unnecessary jerks commonly associated with mild hybrids or cars with engine idle start-stop systems.
The two motors are cooled by automatic transmission fluid, so there is still some maintenance required (no different from a regular automatic transmission fluid change).
The clutch itself is maintenance-free and is designed to last the lifetime of the vehicle, so you don’t need to worry about replacement cost. Although Honda didn’t say what does it mean by ‘lifetime of vehicle’ (service life is dependent on driving habits too), but to car manufacturers, a vehicle’s designed lifespan is typically around 15 years/300,000 km.
The all-new 2020 GN series Honda City won’t be launched until another month or two, and interior photos of the Malaysian specs model have yet to be released.
For now, the car is put on a truck, touring across Peninsular Malaysia, with stopovers at the locations below: