Perseverance Powertrain Design
Updated: Aug 29, 2020
Standing over a noisy diesel, I decided that Perseverance had to do better than the normal power systems, both to achieve the levels of silence I was aiming for and to improve efficiency and hence reduce our carbon emissions.
The wish to be "green" led to a swift consideration of different sources of energy. Gas or pure electric drives are impractical as we don't have space for the gas bottles required to replace a diesel tank and there are not enough places to plug in to recharge a pure electric system. Also, a recharging electric system is, in effect, just pushing the carbon issue onto someone else. Solar power is too unreliable with our dull English weather. Hydrogen is not yet available. So, for the foreseeable future diesel fuel, available at all good marinas around the network, provides the only viable source of energy. Hence we have to start with diesel fuel and end up turning a propeller. Given these start and end points, hybrid systems offer some advantages compared to a basic diesel installation, so I considered those.
There are some hybrid narrowboats on the system today, most of which are of the "Parallel Hybrid" type, where a standard diesel engine is augmented by an electric motor and either can drive the propeller. When the diesel engine is running, it powers the motor (now acting as a generator) to create electricity and this is stored in a battery. Then, the engine can be turned off and the stored battery power is used to drive the electric motor. This appears to have the benefit of allowing the silent operation we are looking for, but there are some drawbacks.
Firstly, the silent operation has to be shared with periods of diesel drive, so this is not silent for all the time. Secondly, when the diesel engine is running, it is very rarely running at full power. Indeed, if the electric motor of, typically, 10kW is large enough to drive the boat, why is the diesel engine three times bigger? (A 43 horsepower engine produces 32 kW). If we consider that a narrowboat on a canal will normally require 3-4kW to drive it along, the diesel engine is running at around 10% of its rated power for long periods. Even allowing for the fact that it is charging the batteries, this still is well below its full power, and hence is running well below maximum efficiency. Finally, the installed system is larger than a standard diesel engine and requires a large battery bank as well, using up much valuable space on the boat.
An alternative form is the "Serial Hybrid" where a diesel engine drives an alternator to generate electricity which drives the motor and charges the batteries. Let's compare it with the parallel hybrid solution.
Silent operation is still shared with periods of, in this case, generating power. However, as the generator is now separate from the diesel propulsion engine we can be far more creative about silencing this engine, and this will form a complete blog post later.
The generator can be sized to meet the average load on the system, and this leads to a smaller diesel engine. Typically something below 10kW compared to the 30kW diesel of a parallel hybrid installation. A small engine is better because we can make sure the engine is always running close to its maximum power thereby maximizing its efficiency. Again, watch out for a more detailed blog on this topic.
Finally, we have separated the motor from the diesel generating source, which gives more flexibility to the installation. For once, we can make use of the key feature of the narrowboat - its length. At this point, somewhere just South of Fazeley Junction as I stood over a noisy, smokey diesel, I resolved that the generator would be in the bow of Perseverance.
This is why, from a simple assessment of the engineering principles, I decided that Perseverance would:
use diesel fuel
have a serial hybrid installation
have a generator in the bow
have an electric motor
Since I arrived at this conclusion with a little thought about the engineering fundamentals, I assumed that the forward looking companies in the marine industry would have had the same ideas, and it should be easy to find the right components to build this boat. How naive was I !
In later blogs I will explain the component selection processes, detail how the generator is being silenced, look at diesel efficiency at partial load and look into batteries. I will also explain why all generator manufacturers exaggerate the power of their generators and why hybrid solutions often fit much larger batteries than they really need.
Oh, and why the photo of a locomotive? This is a Deltic loco, with a unique triangular diesel engine that has 18 cylinders, 36 pistons and three crankshafts. This too was a diesel electric powertrain .