From Honda Accord Hybrid to Koenigsegg Regera

Some of you may wonder why we are talking about Honda Accord and Koenigsegg Regera at the same time. At first glance, these two cars are totally unrelated: the Honda Accord Hybrid has only 196HP combined system power, while the Koenigsegg Regera has 1,500HP. The Accord Hybrid starts from $29,305, while the Koenigsegg Regera carries a price tag of $1,890,000 (close to 1.9 million).

Above image: the Koenigsegg Regera in the 2015 Geneva auto show

In fact, despite the huge difference in power output and price, these two cars have one common feature: they are the only two production cars that connects an internal combustion engine (ICE) directly to the driving wheels. Yes that sounds odd: the ICE propels the wheels directly, no transmission at all.

In this article, I will briefly introduce how such hybrid powertrain works and what is its pros and cons. Part of the information is from the following Honda technical papers:

Development of SPORT HYBRID i-MMD Control System for 2014 Model Year Accord

Development of Motor and PCU for a SPORT HYBRID i-MMD System

Above image: the 2014 Honda Accord Hybrid

Battery Pack: Honda Accord Plug-in Hybrid v.s Koenigsegg Regera

Accord Plug-in Hybrid: Lithium-Ion Battery, 6.7kWh, instant output: 55kW
Koenigsegg Regera: Lithium-Ion Battery, 9kWh, instant output: 500kW

The Regera has around 34% more battery capacity than the Accord Plug-in hybrid, however, its electric motors draw battery power much faster than the Accord. So the Regera’s battery capacity is facing a bigger challenge under heavy track racing conditions.

Engine Direct Drive Hybrid System Explained

See the below illustration, the Honda Accord Hybrid has 3 different working modes (the Koenigsegg Regera will work similarly too):

Above image: 2014 Honda Accord Hybrid i-MMD operation modes

– Pure EV mode: the vehicle is driven by the electric motor only, which draws power from the battery; the ICE is stopped in this mode;
– Hybrid drive mode: the ICE is running, driving the generator; the generator provides electricity to supply to the electric motor, which propels the vehicle; the ICE does not have any mechanical connections to the vehicle wheels. If ICE generates more energy than what the electric motor needs, the excessive power will be used to charge the battery; on the other hand, if the instant power required from the electric motor exceeds what the ICE generates at that moment, extra power will be drawn from the battery;
– Engine drive mode (when vehicle speed > 38mph): the ICE is directly connected to the wheels without a transmission, the vehicle will be driven directly by the ICE with extra assist from the electric motor (if needed).

Above image: Speed and workload range of Honda Accord Hybrid’s 3 driving modes

You can see, the “engine drive” mode cannot be used if the vehicle is moving at a low speed, since that will stall the engine. This is true on both the Accord and the Regera. The engine drive mode kicks in at 38mph for the Accord and 30mph for the Regera. The above image shows each driving mode’s possible speed and workload range .

Above image: illustration of the Koenigsegg Regera’s powertrain

Above image: 2014 Honda Accord Hybrid’s powertrain

Pros of Using Engine Direct Drive (No Transmission)

The reason of not using a transmission, but instead connecting the engine directly to the wheels, is to address two shortcomings in the traditional transmission:

1. Parasitic loss in the transmission components;
2. A brief stop of power transmitting during the period when transmission is changing the gear (not including the CVT)

Pros of Using A Transmission

Most cars with an internal combustion engine (ICE) are usually paired with a transmission. This includes some expensive exotic hybrid supercars, such as the Porsche 918, Ferrari LaFerrari, Lamborghini Asterion and Acura NSX. This is definitely not a coincidence, and there must be some reasons behind it. Especially for the Acura NSX, which uses one ICE plus 3 electric motors. Obviously, Acura has no technical difficulties in using the no-transmission drivetrain scheme like the Koenigsegg Regera, because it has already gained relevant experiences while designing the Accord Hybrid; and it is not for cost too – in fact it saves lots of money by removing the transmission. Therefore Acura has its own consideration on what is more suitable for a hybrid sports car.

Reasons for mainstream auto makers to use transmission on their hybrid sports cars are as follows:

1. To provide maximum torque in low speed and let the car accelerate even if the battery is empty, ICE need to directly participate in providing torque to the wheels. ICE has low torque output in low RPM range (especially near engine idle RPM), therefore it needs different gear ratios to “magnify” the torque output;

2. ICE has different thermal efficiency under different RPM and different workload. By using a transmission, it can let the ICE stay within the most efficient RPM range at any vehicle speed. For example, an ICE outputs 100hp at 2,000 RPM; and it can also output the same 100 horsepower at 4,000 RPM (achieved by varying the amount of fuel injected into the cylinders per cycle). Although the outputs are the same in both cases, fuel efficiencies are different.

The above images shows the engine efficiency relationship for the 2.0L engine used on the Accord Hybrid. The efficiency of each point on the chart (x-axis: engine speed in RPM, y-axis: workload, measured in torque output) is represented by different colors (more red means better efficiency). You can see: for the Honda engine, the best efficiency occurs in mid-range RPM and mid-torque output. Generally speaking, for a given ICE output, mid-range RPM with mid-load is better than low RPM with high load.

3. By using a transmission, a hybrid car is able to run using the ICE only, it will not cause any trouble even if the battery is fully depleted. This is extremely critical for track racing purpose.

Cons of Using Engine Direct Drive (No Transmission)

The first major issue is: when the ICE is connected to the wheels, its spinning RPM will be fixed to the vehicle speed, which means it will be unable to work in the maximum torque/output range at any time.

Secondly, for this design there will be a serious issue if the battery pack is fully depleted. Take the Koenigsegg Regera as an example, its power delivery scheme needs to be really conservative – this is because for speed less than 30mph, it depends solely on the electric motor; ICE will not work for that speed range because low speed will stall the direct connected engine. In short: the Koenigsegg Regera cannot has a fully depleted battery.

In addition, the ICE is unable to stay within the optimal efficiency RPM range. Furthermore, to provide instant high output to charge the battery, the V8 ICE on the Koenigsegg Regera may need to be disconnected from driving the wheels, then rev up to achieve high RPM output (in order to charge the battery as quickly as possible). This situation will occur even at the moment that you need maximum power (because charging the battery has the highest priority). From the previous section, we know the Regera’s battery capacity is not so much larger than the Accord, and Regera’s electric motors are drawing significantly more power from the battery, which will frequently cause the V8 engine to go into the charging mode. This may effectively left the Koenigsegg Regera to be driven only by the 700hp electric motors, frequently.

One more thing makes the situation worse: during hard acceleration, electric motors is drawing energy from the battery at a much faster speed than charging the battery. Therefore under the pure EV drive mode, electric motor output may need to be limited to a lower level to let the battery charging speed catch up with the speed that electricity is drained away. Hybrid cars with a transmission won’t have such tight restrictions, because it can run solely using the ICE without any help from the electric motor.

You may challenge my above discussion since it is just a qualitative analysis. But that is based on what we know from the Accord Hybrid.

Power Management Scheme: 2014 Honda Accord Hybrid

Above image: power management scheme of the 2014 Honda Accord Hybrid

We do not have details of the Regera’s power management algorithm; however, Honda did show us how the power is managed in the Accord Hybrid. I consider it to be very representative.

See the above image, you will find most of the time, Honda does not allow the ICE to be connected to the wheels (ICE only drives the wheel during high speed acceleration phase); In high-speed hard acceleration, when the ICE is participated in driving the wheels directly, the battery is also in the discharging mode, which means such output level cannot be maintained for longer period, and the battery charging phase (which draws power from the ICE) will follow immediately.

Therefore, Acura uses a 7-speed DCT on the 2016 NSX for these major reason: it not only makes it possible to keep the engine RPM within the optimal efficiency/torque/hp region, but also has higher possibility to supply the maximum system power whenever you need it. The same reason applies to other hybrid sports car like the Porsche 918, Ferrari LaFerrari and Lamborghini Asterion.

For the Accord Hybrid, since its main application is for daily commute, and it is rarely used in track racing, therefore it is highly unlikely that you will need frequent hard acceleration. So it is suitable for the hybrid powertrain to NOT use a transmission, and connects the engine directly to the wheels.

Summary

For Koenigsegg Regera, the “true” reasons why it omits the transmission in the hybrid system, from our point of view, may be due to:

– it already packs a 5.0L twin-turbo V8, a generator, and three electric motors in the back of the car; if adding another transmission, the tail of the Regera will be over-weighted compared to the front;
– Adding a transmission into the powertrain makes the whole system more complicated. It requires huge amount of capital, a larger R&D team and significant technical knowledge reserve to handle the design, testing and manufacturing, which Koenigsegg may be lack of.
– Patent issues.

Had Koenigsegg used a transmission in the hybrid powertrain, the Regera can get similar performance, with smaller peak output and better fuel efficiency.

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