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Preliminary Design, Drive-Cycle Simulation and Energy Analysis of a Hybrid Transit Bus

Modern metropolises are increasingly affected by air quality problems. Transportation is one of the largest sources of several pollutants emissions, such as nitrogen oxides (NOx) and carbon monoxide (CO). Today in the EU, vehicles’ emissions are strictly limited by Euro 6 norm—Euro VI for heavy-duty vehicles—which is periodically upgraded. To match such limits, manufacturers are forced in developing new technologies to perform new sustainable vehicles design strategies, such as EVs and HEVs. Present work’s aim is to provide the design of series-hybrid urban transportation bus, equipped with a novel thermal power unit, namely a small gas turbine, to exploit its cleaner combustion process in comparison with an ICE. The control logic is described, while the main drivetrain components are chosen, and suitable models from suppliers are selected as well. Then, some simulations of the resulting vehicle are performed on opportune drive cycles, using Advisor, a free software based on Matlab-Simulink environment, published by US’ National Renewable Energy Laboratory (NREL). Two different final configurations are environmentally and economically analysed, with the thermal power unit being respectively fuelled by compressed natural gas (CNG) and liquefied petroleum gas (LPG). Both satisfy the Euro VI norms, showing a substantial emission reduction (-89% and -43% in CO and THC releases respectively) in comparison to pollutants’ threshold values.


Introduction
The present paper has the purpose to provide the design and drive-cycle simulation of a hybrid urban transit bus, to quantitatively determine how much emission would be saved within the Euro VI boundaries. Moreover, an economic analysis of the designed drivetrain impact is performed in terms of fuel consumption and transport costs, in comparison with two traditional diesel-ICE analogues vehicles.
This work is developed in conjunction with the Department of Mechanical and Aerospace Engineering of "Sapienza" University of Rome, Italy, and it takes its origins from several previous studies, cited in the GTHV configuration paragraph.

Pollution by Road Transport Sector
Air pollution is the chemical alteration of atmosphere. It results from the release of compounds that are not included in air natural composition, as well as species that are commonly included in atmospheric gaseous mix but in different quantities. Emissions reduction theme is one of the biggest challenges of our era [1][2][3][4], and it is a priority point of automotive industry. Transportation sector is one of the major responsible in air pollution, especially when referred to big cities areas. It is possible to observe from Fig. 1 how large is its impact on each pollution agent of total emissions. Road transport sector results to be the heavier in NO x emissions with about 40% of total, and the second in CO and PM2.5 releases with respective approximate shares of 20% and 10%.

NO x Issue
Nitrogen oxides (NO x  Total hydrocarbons (THC) emissions include mostly unburned HCs (UHC) that are fuel portions emerging from the combustor, as well as the products of fuel thermal degradation into lower molecular weight species. Generally, the factors that influence CO emissions also influence UHC emissions and in much the same manner [11]. The current Euro emissions limits for THC include methane and non-methane hydrocarbon (NMHC) emissions. The main reason for including methane is the fact that it is a strong greenhouse gas.

Euro VI normative
The "Euro VI", when referred to high-duty vehicles (with a mass greater than 2,610 kg), is the current emission normative in Europe, meaning that at present, only Euro VI vehicles can be sold in the EU. Since the 70s, the increasingly tighter emission limits have led to the introduction of new technologies, consequently there have been some significant reductions in vehicle emissions. One need only think that a Euro 6 diesel car must release 97% less PM than a 20-years-older Euro 1 vehicle.
Although results are in evidence, emission reductions from road transport have been lower than originally predicted, partly because transport has grown more than expected, and, for certain pollutants, partly due to a larger than expected growth in diesel vehicles, producing higher NO x and PM emissions than petrol-fuelled vehicle, but, most of all, it is widely accepted that real-world emissions of NO x , particularly from diesel vehicles, generally exceed the permitted Euro emission standards.
Actually, real-world emissions are a crucial point for current legislation centres and observatories, as many studies [12][13][14][15][16] demonstrate the inability of test procedures that have been performed for many years to correctly determine the real vehicles driving loads and sequences. The reason of the failed correspondence between real and tested emissions is that the NEDC test cycle, previously used for these certifications, does not capture the full range of typical real-world driving operating conditions of the engine map. These inaccuracies have been fixed in the latest Euro 6 and Euro VI directives upgrades that came into force from September 2017. Particularly in the Euro VI standards, old drive-cycles have been replaced by both the World Harmonized Stationary Cycle (WHSC) and the World Harmonized Transient Cycle (WHTC) that were defined alongside with United Nations Economic Commission for Europe (UNECE).
To follow, EURO VI limits ( Fig. 2) as specified in its latest review (July 2017), and a comparison between NEDC and WHTC profiles. From Fig. 3, it can be observed how the newer cycle has a higher level of variability that reflects its real-drive conditions. Therefore, it origins higher emissions and energy request values to the same vehicle body.
To achieve all design goals of an urban transit bus, the procedure will be focused on: Ensuring adequate vehicle's performances in following common real-drive cycles; Matching the most recent emission standards, namely Euro VI limits specs from 2017, July; Minimizing fuel consumption; Analysing the transportation cost by the hybrid drivetrain.

Hybrid Configuration
The chosen design configuration is based on a gas-turbine (GT) power generator as thermal unit of the drivetrain. GT can be ignited by different fuels in two studied cases: compressed natural gas (CNG), and liquefied petroleum gas (LPG).   Hybrids (series configuration) technical strength is to rely on the electric power unit (PU) to absorb the highly variating part of the power request, letting heat engine handle the constant load. It can be also possible to use the thermal PU as a generator for the electric energy storage system. This circumstance is highly competitive in terms of drivetrain efficiency, as it lets the thermal PU work at a constant optimal operating point, that is the minimum specific fuel consumption condition. Thus, the electric storage system operates like a "power damper", and efficiency level is much less deteriorated by load variations [17].
Finally, electric machines are fully reversible, so they can act in both power-release, or regeneration mode by wheel braking, which provides a further efficiency improvement. However, braking kinetic energy recovery system (KERS) must respect the bounds of the maximal power that can be accepted as input for electric storage system. This constraint prevents from a full braking kinetic energy recovery by KERS.

GTHV Design Configuration
The design carried out in the present thesis is based on previous works and studies developed at the Mechanical and Aerospace Engineering Department of Sapienza University, all of them dealing with the same kind of gas turbine hybrid vehicle (GTHV) configuration [18][19][20][21][22][23][24]. GTHV is a series hybrid drivetrain where the heat engine is constituted by a small gas turbine (GT), or microturbine (MT), while only the electric PU drives the vehicle (Fig. 4).
From a thermodynamic point of view, GTs run on Joule-Brayton open cycle, which uses constant pressure heat transfer processes with compression and expansion phases in between. Working fluid is air, and it is continuously drawn and compressed in the compressor. Then it moves into the combustion chamber, where it is burned with injected fuel, in a continuative combustion process. At this point, the high-enthalpy flow goes through turbine's blades channels where the fluid is expanded and cooled, while useful power is collected. At the end of the cycle, exhaust gas flow is usually directed to a regenerator to operate the inlet-air preheating.
GTs do not have a very performing response to power modulation and it is more convenient to set it operating at a fixed-speed point of the map. Typically, these engines reach the best efficiency near the full load conditions, at very high speed (60-90 krpm and higher).
A quick summary of some GTs engines strengths: The elevate rotating speed leads to higher power-to-weight ratios, against traditional ICEs, which means a more compact, lighter machines design; There are less moving parts, as the only one in a GT is the rotor; Combustion is continuous. That means it is well controllable and much more effective in fuel oxidation process. Consequently, CO and THC emissions are substantially lesser than those released by a reciprocating ICE; Continuous combustion can be even traduced in a very mild dependency by the type of fuel, against a reciprocating ICE. Therefore, a GT can operate with many fuels with almost the same performances.
Main issues from a GT are linked to the more complex design instead, that means higher design, manufacturing and materials costs. Furthermore, the cited hard capability to regulate power is a bad point from GTs, but in a series of hybrid configuration, where to heat engine is asked to operate at a fixed, optimal point, this concern is negligible.

Control Logic
The electronic vehicle management unit (VMU) has the mission to control and operate on all vehicle's parameters, to reach the desired dynamic drivetrain performances. The VMU must have available all the instantaneous values of thermal, mechanical and electrical parameters, in a processable format. These

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Base Parameters Definition
Referring to a typical standard (not articulated),

Required Power
Under the specified conditions and with the drag and rolling resistance coefficients from previous Table  1, it is possible to write down the expressions and the quantitative values of dynamic resistances the vehicle must exceed to provide motion. Then, the value of required power by the actuator would be a direct consequence. From definitions [26]:

· Kw
where, R in = inertial resistance R roll = rolling resistance R air = air resistance R grad = road gradient resistance α = ground angle c i = 1; inertial and rotating masses coefficient (its values exceeding unit are mostly due to clutch rotating masses). In GTHV transmission this effect is negligible and c i is equal to 1 ρ air =

Electric Motor
Electric motors constitute an optimal design choice for a vehicle drivetrain, as their torque and speed characteristics well satisfy the vehicle dynamics basic requirements, especially in applications where there is no need of extremely high-speed rates, like in the case in exam. The recent HEVs employ AC and brushless motors, which include induction motors (IM), permanent magnet motors (PM), and switched reluctance motors (SRM). IM and PM are commonly retained to be the better choice for automotive applications [31][32][33].
The designed vehicle utilizes an AC-PM motor, as this kind of machine provides the maximum power-to-weight ratio among EMs technologic supply. Of course, the EM must satisfy the mission power request set by the peak value of 200 kW. To satisfy that, it has been chosen a model from AVID Technology Ltd, a UK-based company which produces powertrain components for hybrid and electric vehicles (Fig. 6 and Table 3). To follow, there are reported the chosen model characteristics, from Ref. [34]:     [35]. For this specific application, the ESS is demanded to provide a peak power of 200 kW and a rated capacity of 60-70 kWh at least. For better facing the design of a suitable battery pack, it has undertaken a precious information exchange with the Italian ESS producer Kaitek srl (RE, IT). The supplied data claim that such a Li-ion cell with LiFePO 4 , can observe a 1.5 C continuous rate of discharge, which rises up to 3 C for brief peaks (10 s). About charge rate instead, its rated value is set to 0.5 C continuously and 1 C for peaks. Moreover, it has specified the indicative cost of these devices, namely about 500 €/kWh, which also includes all cables, BMS, boxes, charging devices. Thus, the proposed preliminary design sets an ESS constituted by a series-connection of 27 prismatic cells by Kaitek, each one characterized by 24 V and 100 Ah. In this way, the ESS nominal voltage output is maximized to 648 V, to fulfil the constraint about motor's minimum voltage input, at the same time minimizing currents and conduction losses. The following table (Table 4) reports the ESS key-parameters.    [43]. Its main objective is to predict, through many output data, the performances of still-not-built vehicles.

Vehicle Setup
Two preliminary steps must be executed before launching a simulation on Advisor: Defining the vehicle, through the set of input data from its components; Assigning the mission, that is the drive-cycle. The first screen by Advisor allows the user to compose the desired drivetrain through many different predefined or custom schemes (parallel hybrids, series, fuel cells, traditional ICE, and so on). For each scheme, every component of the vehicle is defined by a script on Matlab editor which contained all its specifications. At this step, all components scripts must be opportunely modified and renamed, to be representative of the chosen elements.
During the process of scripts creation/modification, the point was focused on PUs and battery pack, for matching the real conditions and specifications of these components.
The fuel converter, that is the GT unit, is modelled using data from one of the major suppliers, Capstone Microturbine Corporation. Electrical efficiency is set to 0.32. In the vehicle setup from Fig. 10, GT and generator items are controllable as distinct components, while electrical efficiency of GT group is the aggregate data for the two components. Thus, it has set the electric generator efficiency to 1, while all losses are accounted in the GT (fuel converter) component script. Fuel consumption is given in terms of power input from C200 datasheet [38], and it is equal to 2,400 MJ/h (MJ of Higher Heating Value, HHV). This means that fuel consumption values (in g/s) are different for fuels with different HHVs. Not being available the entire map-range data, fuel consumptions for GTs (CNG and LPG fuelled) are set equal to values from datasheets (scaled in power) 2 , right in the operating point. This is an acceptable choice in a constant, full-load control logic: The electric motor has been reduced in overtorque factor, to provide the maximum power value of 110% its nominal power. This means a maximum motoring peak power of 206.8 kW. Peak power limit is necessary to match the ESS instantaneous discharge limit, avoiding current overloads.
The ESS uses data from Table 4. Notice that module's internal resistance values and its variations with SOC were not available, thus these variables have been referred to opportunely modified data from a default file in Advisor directories (ESS_LI7_temp.m), that is representative of a Li-ion module.
Other remarkable vehicle's setup operations: Powertrain control has been set to let the MT provide to the generator, in the described On-Off logic, a constant power value, equal to its maximum (full load); Gear ratio has been reduced (-12.5%), after a brief by-trial optimization; Vehicle body data, as well as wheels' ones, have been set in the respective files; The total mass of the vehicle is defined to match the maximum tolerated value by axles, as reported in Table 1. Therefore, the total mass is set equal to 19 tons, including a little further oversizing. It represents the full-load operating conditions of an urban transit bus.

Driving Cycle
In the simulation setup screen, there are all the 2 Datasheets of C65 and C200 MTs from Capstone [37,38] report respectively 919 MJ/h and 2,400 MJ/h as values of fuel consumption. These correspond to different values in g/(s·kW) between the two machines, that are respectively (CNG version) equal to 0.0747 and 0.0634, that is a 15% difference (i.e. the larger is a machine, the lesser is its fuel consumption, per unit of output power). To account for this circumstance, it has calculated the weighted (on power output) average value of such a data for a 100 kW turbomachine, that is 0.0717 g/(s·kW) for the CNG fuelled MT. Derived values in (g/s) are reported in Table 6. which has b as well as umber of c Such an adjustment is largely acceptable for this kind of application. For simulating a real-drive scenario for an urban transportation vehicle it has been set the number of repetitions equal to 24, to simulate drivetrain's performances over a realistic 12-hours continuative operating interval. It is closely representative of a daily 8am-8pm turn. Finally, initial conditions have been set, by imposing 0.8 as starting SOC value.

CNG Results
The last screen from Advisor reports the simulation results, through tables and selectable graphs that show the behaviour of every key-parameter, by any component, along the entire drive-cycle. It is remarkable the good level of coherence between the imposed and the achieved speed, as well as the correct shape SOC variations [44][45][46], limited to the specified range 0.4-0.8, and the GT emissions, whose result is stable, in correspondence with turbine's ignition. However, notice that the frequency of charge-discharge cycles of ESS is quite high, as it requires to be recharged 10 times per day (12 hours driving turn). Final SOC value is maximum and the vehicle is ready to restart another mission after the refuelling. Table 7 reports all the key-values resulting from the simulation of the CNG vehicle.
The table highlights the substantial saving in emissions, with respect to the tight normative from EU. This means that such a hybrid configuration could become an innovative alternative technology to face the recent (and future) increase in norms' strictness. It remains to consider the economic impact of such hybrid drivetrain in terms of fixed production costs, as well as the transportation cost it would require, in comparison with analogues traditional ICE vehicles.

LPG Results
The LPG vehicle simulation differs from the CNG one by fuel consumption and emissions, while the other performances are substantially equivalent. LPG produces more NO x than CNG, its energy density is higher (J/m 3 ), but its specific energy is lower (J/kg). Therefore, it is expected a higher NO x emission level, and a lower consumption in volume of fuel. Results confirm the expectations, as NO x emissions increased,   while fuel volume requirement is lower. Table 8 resumes LPG vehicle simulation results. Comparing Tables 7 and 8, CNG clearly emerges as the better fuel for this kind of application, as it provides the lower pollutants emissions and the lower per-km expenditure. Of course, it would need a larger and more technological tank, but as it is shown in Table 1, such solutions are available for these vehicles.

Daily Cost: GTHV vs. ICE Comparative Analysis
For a comparison, there have been considered two ICE-diesel models, one is the Irisbus Cityclass 491 from Table 1, which is an old model, but still very diffused as a public transportation bus in the municipality of Rome. The second is a modern Mercedes Citaro Euro VI [47].
About costs from fuel, the CNG fuelled GTHV results in the most convenient solution, at the current fuel price in Italy, which for diesel is equal to 1.45 €/L [48]. The LPG version instead proved more convenient than the Irisbus, but less than the Mercedes. These results seem to proof the unit cost benefit in advantage of GTHV (CNG) bus, but for this kind of vehicle it must also consider the ESS maintenance cost which is one of the most critical points in HEVs. Batteries suffer degradation after a long and frequent service and it must consider their periodic substitution. In this case, claimed battery life, from Table 3, is 3,000 cycles, while its unit cost amounts to 500 €/kWh. To estimate the ESS daily cost it is possible to write: where, frequency is the daily number of charge-discharge cycles (=10 cycles/day); CAP represents the capacity of the ESS (= 64.8 kWh); LIFE is the number of cycles after which it could degrade (= 3,000 cycles); c unit is the unit cost (= 500 €/kWh). Executing this calculus for the GTHV vehicle, it results the value of 108 €/day, to sum to the fuel costs.
This scenario compromises the GTHV economic convenience, as its less expensive version-CNG-reaches a daily cost equal to 261 €, namely 69% more than the recent Mercedes, and 28% higher than the Irisbus. This analysis has been conducted with a conservative approach, as for example there has not been considered the ESS life extension benefit by the limitation in SOC operative range, as well as the probable unperfect matching between constructors' and real consumption data for the antagonist vehicles. Moreover, there have not been accounted for any government's incentives due to emissions reduction, and the simulated drive conditions are constantly at maximal load condition, while for a transit bus the real load varies a lot, due to passengers' hourly fluxes. Thus, there is a margin to lower the gap between GTHV and modern diesel bus costs, making the former more competitive.

Conclusion and Future Approach
The design of the gas turbine hybrid bus, has led to simulations results that reveal a substantial emission level reduction of the CNG fuelled version of the GTHV bus. This configuration meets the normative constraints by far. Data from simulations (Table 7) testify a fraction of Euro VI (2017 update) admissible limits (see Fig. 2) equal to 56% for NO x , 11% for CO, 57% for THC; corresponding respectively to 44%, 89%, 43% savings. Moreover, the burning of gaseous fuels like CNG (and LPG) emits a negligible PM fraction.
The version fuelled with LPG does not reach the same savings in emissions, due to the worse NO x level (91% of Euro VI limit), and at the same time it suffers higher operating costs. Therefore, although CNG version requires a larger, high-pressure tank (200 bar), it appears to be the best and more promising choice for implementing further studies and design about this kind of urban transportation vehicle.
The reduction in emissions of CNG GTHV is attributable on both the installed MT as the thermal PU, and series hybrid configuration adopted. The former, guarantees a better, continuous, more effective combustion if compared to a reciprocating ICE; the latter, lets the electric motor absorb the varying part of load, with the high efficiency of any electric machine, while the GT works at fixed point (rated power) to maximise its efficiency. However, this configuration implies the design of large electrical powertrain, that means higher costs for the electric storage system. Unit transportation costs are approached in a comparative study between GTHV bus and two real models, an old Irisbus Cityclass 491, and a newer Mercedes Citaro, Euro VI certified. It emerges that, only for fuel's cost, the tested vehicle is the more economic (Table 9), with the expense of 153 €/day against 203 and 154 €/day from Irisbus and Mercedes respectively. However, adding ESS maintenance cost (substitution after a lifetime), the GTHV would require 261 €/day, well over its cited opponents.
From these calculations, it results that, if hybrid technology's environmental benefits are in evidence, its higher costs are still the most critical limit in HEVs development. The economic analysis has been carried out with a very conservative approach, thus it is expected to obtain more promising economic results in a more refined study.
In a future potential approach to a new GTHV bus design, it is recommended to observe and study the vehicle performances with attention on: ESS design. The battery pack could be slightly oversized to a major value respect to the actual 64.8 kWh, to reduce both depth and frequency of charge-discharge cycles, to extend the ESS lifetime, reducing its daily-substitution costs. Of course, in this case would occur a higher installation investment, thus such balance must be optimized, with the help of more detailed studies and data about batteries behaviour.
Control logic. A more refined control of energy fluxes could lead to further emissions and cost savings.
It may test a Charge Sustaining (CS) logic to replace the actual Charge Depleting (CD) strategy. In this way, the frequency of ESS charge-discharge cycles could be minimized, and thermal PU can be downsized for supplying a reduced power surplus, as the SOC must not be increased but kept constant.
GT downsizing. It has been noticed that a WHTC drive-cycle from real-drive condition requires the average power of about 40-50 kW to drive a 19-tons heavy-duty bus like the tested one. This means that a smaller (65 kW) microturbine may be sufficient, for supplying enough power for both moving the vehicle and recharging/sustaining batteries, when SOC is at its minimum. This allows downsizing the thermal powertrain, with expected further savings in drivetrain building costs. Moreover, at mission equality, a turbine's downsizing should also provide a lower emitting vehicle (as datasheet's claimed emission factors for Capstone's C65 are lower than those by C200), but it would require a higher continuous ignition time, namely higher fuel consumption, given also that efficiency in turbines is generally a direct function of rated power. Finally, GT downsizing is another agent in ESS cycles frequency reduction, as the power surplus for battery recharge operations is reduced, consequently it would increase the ESS recharging time.
Studies on vibrations and acoustic impact of an on-board generating microturbine system.