Calculating The Most Efficient Method Of Power Transmission

Internal combustion engines (ICE) show different efficiency levels at different loads and rotational speeds. This not only connected with processes of combustion, but also with technical layout of an engine.

The following table shows the efficiency of tested Skoda Fabia engine with RPM on the vertical axis and Output torque and load on the horizontal axis.

Table 1 “ICE efficiency”

% load	9	18	27	36	45	55	64	73	82	91	100


output torque (Nm)	15,6	31,2	46,8	62,4	78	93,6	109,2	124,8	140,4	156	171,6
RPM
500	13,53	18,72	22,35	25,02	27,08	26,45	27,4	25,57	28,77	28,77	28,77
1000	13,54	20,55	24,62	26,92	29,13	28,95	29,32	29,32	28,77	28,77	28,77
1500	13,54	20,37	23,98	27,08	29,51	31,53	32,56	33,6	32,88	28,77	28,77
2000	13,95	20,92	24,75	27,56	30,09	31,97	33,85	33,85	33,36	31,53	28,77
2500	14,85	21,71	25,86	29,13	31,1	33,6	33,85	33,85	33,85	32,42	28,77
3000	15,87	22,68	26,61	29,7	31,53	32,42	33,36	33,85	33,85	31,97	28,77
3500	17,7	24,36	28,77	29,51	29,89	30,98	31,97	33,36	33,36	31,53	28,77
4000	15,87	23,25	26,1	27,24	29,32	30,09	31,53	32,65	31,53	31,1	28,77
4500	15,34	21,12	23,58	26,15	28,24	29,7	30,49	30,69	30,36	30,09	28,77
5000	12,11	17,98	21,92	24,88	27,08	29,13	29,25	29,7	29,28	29,13	28,77
5500	10,96	16,21	19,67	22,68	25,29	28,07	27,73	28,07	28,42	28,59	28,77
6000	10,96	15,66	19,18	22,13	24,75	26,46	27,73	28,07	28,42	28,77	28,77

From the table above we can see that tested engine performed at peak efficiency (33,85 %) in the range or 2000 to 2500 RPMs and under approx. 55-90 percent load.

Depending on circumstances engine is loaded in different ways, for example: it works on idle most of the time in traffic jam, where it’s efficiency is abysmal at 13,53% at creeping speeds and 0-3% stationary (engine burns a lot of fuel not doing any useful work except keeping lights and air conditioner on).

On the highway engine typically operates at its most efficient range of RPMs and loads, therefore MPG on any ICE on the highway is the best.

Based on these facts, we can deduct a calculation of typical improvement of the efficiency of a vehicle overall from the implementation of hybrid drive. In this case by hybridization, we understand full electric drivetrain and ICE working only as a generator.

Combined drivetrain efficiency of a regular vehicle equals the product of efficiencies of: (ICE on a given RPM and load) and (gearbox+4wd+reduction drives). 4wd because we compare apples to apples

The efficiency of hybrid module can be calculated with multiplication of efficiencies of: (electric engine)*(it’s reduction drive)*(battery in)*(battery out)*(generator).

Let’s discuss several scenarios:

On the highway ICE in a regular vehicle works at peak efficiency, so the improvement will depend solely on the difference in efficiencies of gearbox and combined module (electric engine and its gearbox). So the overall module efficiency must be 87% and above to beat modern vehicles in this case.
In city the situation is the opposite for regular cars, at least half of the time ICE works on idle because of waiting on the lights or staying in a traffic jam, so the combined efficiency of the hybrid module can be as low as 65% to beat ICE it this case.
Combined cycle is typically calculated as mean of the other two.

However, these figures do not include benefits of energy recuperation, available for hybrid vehicles (it’s the energy that can be collected from deceleration, otherwise lost as heat). Averaging these figures is a controversial topic because everybody uses cars differently, but on the bigger picture the recruitment of energy recuperation gives approximately 10-15% difference of energy expenditure.

So that means that any hybrid vehicle, combined drivetrain efficiency of which is lower than 75% but higher than 55%, is more efficient than its ICE counterpart in cities, and if the combined efficiency is greater than 75% – it is more efficient everywhere.

Calculating The Most Efficient Method Of Power Transmission

Hybrid

Full-electric